All about Astronomy Thread - Our Expanding Universe: Age, History & Other Facts

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  1. Dr. AMK

    Dr. AMK The Strategist

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    Cosmic Dawn: Astronomers Find Fingerprints of Universe's First Stars
    https://www.space.com/39837-first-stars-universe-fingerprints-dark-matter.html
    By Mike Wall, Space.com Senior Writer | February 28, 2018 01:01pm ET
    [​IMG]
    An artist's illustration of what the first stars in the universe may have looked like.
    Credit: N.R. Fuller, National Science Foundation
    The cosmic dark ages lasted no more than 180 million years.

    Astronomers have picked up a long-sought signal from some of the universe's first stars, determining that these pioneers were burning bright by just 180 million years after the Big Bang.

    Scientists had long suspected that dawn broke over the cosmos that long ago; theorists' models predict as much. But researchers had never had the evidence to back it up until now. Before this new study, the oldest stars ever seen dated to about 400 million years after the Big Bang. [The Universe: Big Bang to Now in 10 Easy Steps]

    "This pushes our knowledge of when and how stars formed to earlier times in the universe," said study lead author Judd Bowman, an astronomer at Arizona State University's School of Earth and Space Exploration.

    These very ancient stars were trailblazers. Though they coalesced from primordial hydrogen and helium, they set in motion a continuing process of star birth and death that ended up, over the eons, seeding the universe with heavy elements — the stuff that rocky planets like Earth are made of.

    "If you look at our cosmic origins," Bowman told Space.com, "the bottom rung of that ladder is this process of the first objects forming and enriching the medium to make everything else possible."

    In addition, the signal that Bowman and his team found was surprisingly strong. It was so strong, in fact, that it hints at a possible interaction between mysterious dark matter and the "normal" stuff that makes up the stars and you and me and everything else we can see in the universe.


    Sifting through the noise
    The further back in time you go, the harder it is to spot stars directly, using instruments such as NASA's Hubble Space Telescope. For starters, there are fewer and fewer stars to find. And until about 500 million years after the Big Bang, the universe was suffused with neutral hydrogen atoms, which are good at blocking light. (Radiation from the first stars eventually split these atoms into their constituent protons and electrons, creating a more transparent ionized plasma, but this took a while.)

    So, Bowman and his colleagues took an indirect route, searching for the fingerprints these early stars likely left on the cosmic background radiation(CMB) — the ancient light left over from the Big Bang. The stars' ultraviolet radiation, the idea goes, would excite hydrogen atoms into a different state, causing them to absorb CMB photons.

    Theoretically, this dip in the CMB signal should be detectable. So, the team built, calibrated and tested a radio antenna the size of a kitchen table — a project they called Experiment to Detect the Global EoR (Epoch of Reionization) Signature (EDGES), which was funded by the U.S. National Science Foundation (NSF).

    [​IMG]
    The EDGES ground-based radio spectrometer, at CSIRO’s Murchison Radio-astronomy Observatory in Western Australia.
    Credit: CSIRO Australia
    Then, they set the equipment up at the Murchison Radio-astronomy Observatory (MRO) in Western Australia. The MRO is in an extraordinarily radio-quiet area maintained by the Commonwealth Scientific and Industrial Research Organization, Australia's national science agency.

    The radio-quiet aspect of the site was key, because modeling work suggested that the signal Bowman and his colleagues were looking for overlapped with frequencies on the FM radio dial. And the researchers already had to contend with all of the Milky Way's booming background radio noise. [Stunning Photos of Our Milky Way Galaxy (Gallery)]

    "There is a great technical challenge to making this detection," Peter Kurczynski, the NSF program director who oversaw funding for EDGES, said in a statement. "Sources of noise can be 10,000 times brighter than the signal. It's like being in the middle of a hurricane and trying to hear the flap of a hummingbird's wing."

    But EDGES picked up that tiny flap, spotting a dip that's most intense at a frequency of about 78 megahertz. Hydrogen emits and absorbs radiation at a wavelength equivalent to 1,420 megahertz, so the signal EDGES detected had been "redshifted" — stretched to lower frequencies by the expansion of the universe. The extent of this redshift told the team when those CMB photons were absorbed: about 180 million years after the universe's birth.

    Bowman and his team reported these results today (Feb. 28), in a study published online in the journal Nature.

    "These researchers with a small radio antenna in the desert have seen farther than the most powerful space telescopes, opening a new window on the early universe," Kurczynski said.

    The EDGES signal petered out less than 100 million years later, probably because X-ray light emitted by supernovas, black holes and other objects had heated up the hydrogen atoms significantly by that point, Bowman said.

    A timeline of the universe, updated to show when the first stars emerged (by 180 million years after the Big Bang).
    Credit: N.R. Fuller, National Science Foundation
    Dark matter involved?
    The signal EDGES found was about twice as strong as the team expected. There are two possible explanations for this surprising intensity, Bowman said: Either the radio background was quite a bit stronger in those early days than scientists had thought, or the hydrogen gas was significantly cooler.

    The study team leans toward the second possibility, because it's tough to imagine a process that would increase the radio background to the necessary levels, Bowman said. It's also tricky to figure out what may have cooled down the hydrogen, but there is a promising contender: dark matter, the mysterious stuff that makes up 85 percent of the material universe.

    Dark matter neither absorbs nor emits light, making it impossible to see directly (hence the name). Astronomers have inferred the substance's existence from its gravitational effects on "normal" matter, but they don't know what dark matter actually is. Most researchers think it's made up of as-yet-undiscovered particles, hypothesized specks such as axions or weakly interacting massive particles.

    In a separate study in the same issue of Nature, astrophysicist Rennan Barkana, of Tel Aviv University in Israel, suggested that cold dark matter may have sucked away energy from the hydrogen gas, cooling it down. If this happened, "the dark-matter particle is no heavier than several proton masses, well below the commonly predicted mass of weakly interacting massive particles," Barkana wrote in his study.

    If Barkana is right, Bowman and his team have gotten a look at some exotic physics and uncovered an important clue about the nature of dark matter. [Gallery: Dark Matter Throughout the Universe]

    "We've been looking for so long for anything that can tell us more about what dark matter might be," Bowman said. "If this indeed gets borne out and continues to be confirmed — that the detection is real, and Rennan's hypothesis is real [and] is the best explanation — then this might well be the first key to advancing our knowledge of what dark matter really is."

    Next steps
    Speaking of confirming the detection — that's the immediate next step in this line of early universe research, Bowman said. He and his team spent about two years validating their find, ruling out all possible alternative explanations. But for the discovery to be rock-solid, another research group needs to spot the signal as well.

    If that happens, astronomers can mine the signal for more information, Bowman said. After all, now they know where to find it.

    For example, further study by sensitive radio-telescope arrays should reveal more about the nonstandard physics hinted at by the signal and more about the properties of the universe's first stars, he said.

    "Also, we would expect that we can eventually start to discern when the very first stars transitioned into the second-generation and later stars that were built out of gas that had heavier elements in it," Bowman said. "I think all of that then gets tied into the origin and formation of galaxies as a whole."
     
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  2. Dr. AMK

    Dr. AMK The Strategist

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    Researchers Discover Monstrous Supermassive Black Holes
    https://www.pbs.org/newshour/science/monster-black-holes-found-in-distant-galaxies
    blackhole_v3_slideshow.jpg
    Researchers have discovered a monster black hole that appears to be the most massive found to date, as massive as 21 billion suns. This is one of two black holes found in elliptical galaxies some 300 million light years away — both believed to be the biggest yet.

    A black hole occurs when gravity causes an object to collapse under its own weight to a point, creating an object that is fantastically small, yet tremendously dense. They are voracious eaters, sucking in everything they can absorb, and once formed, the gravitational pull is so strong that nothing — not even light — can escape. This region where nothing can escape – the point of no return – is known as the “event horizon.”

    These black holes have event horizons as large as five times the size of our solar system.

    “That tells you how scary it is – how big that region is in space from which nothing can get out,” said Chung-Pei Ma, a professor of astronomy at University of California, Berkeley, and one of the authors of the study, which was published online Monday in the journal Nature.
    The biggest of the two black holes weighs as much as 21 billion suns, though its data contains more uncertainties than the littler one, which weighed in at about 10 billion suns, said Nicholas McConnell, a U.C. Berkeley graduate student and the study’s first author. To put the sheer size of these in perspective, the previous record holder, found 33 years ago in the galaxy Messier 87, has a mass of about 6 billion suns.

    The mass of a black hole tends to correspond to the mass of its galaxy. Generally, the bigger the galaxy, the bigger the black hole at its center, a sign that the formation and evolution of a black hole and parent galaxy are intimately related, Ma said.

    But these black holes are bigger than they should be given the size of their host galaxies. Ma said that black holes from neighboring elliptical galaxies most likely fused together when their galaxies merged. And they may have gained additional mass by sucking in surrounding gas.

    That something different is occurring with these giants warrants further study, said Kevin Schawinski, a Yale university astrophysicist. “We actually think that this link between galaxies and the black holes that live at their center is a really fundamental relationship. And the fact that we now see in the most massive galaxies hints of a departure – that’s interesting.”

    Scientists calculate the mass of black holes by studying the matter swirling around them. In this case, researchers used state-of-the-art spectrographs on Hawaii’s Gemini North and Keck 2 telescopes to determine the speed and motion of stars orbiting around the black holes. The more massive the black hole, the faster objects will orbit around it.

    McConnell compared the technique to a police officer using a radar gun to determine how fast a car is speeding down the highway. While the radar reflects off the car back to the police officer, the starlight intrinsically moves toward us. But it’s light from a source moving toward a detector. “Within that light, the wavelengths give you information about the speed at which the object is moving,” he said.

    A next step, Ma said, will be to simulate how these galaxies and their supermassive black holes grow with time by modeling on supercomputers how these holes merge and feed.

    Ma’s team had been analyzing data from the past four years when McConnell alerted her to numbers indicating a potential monster black hole, she recalled.

    “When you see a number like that, your first reaction is, ‘Ooh, could that number be wrong?'” she said. “But the number didn’t want to go away. It was pretty consistent; the statistics really preferred a big black hole. That was the information the stars were telling us: We’re orbiting a big, black hole that you can’t see.”

    The stars measured were a few hundred light years away from the black hole — not close enough to make the job easy, McConnell said.

    “But it is close enough that we’re getting a signature that can’t be explained only from dark matter or only from other stars. We’d love to get closer, to make the uncertainties of our measurements go down. And we’ve measured starlight close enough to definitely say that we’ve discovered something very massive.”

    This post has been updated since its original version.

    Photo credit: An artist’s rendition of stars moving in the central regions of a giant elliptical galaxy that harbors a supermassive black hole. Image by Gemini Observatory/AURA artwork by Lynette Cook
     
    Last edited: Mar 10, 2018
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  3. Dr. AMK

    Dr. AMK The Strategist

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    Dr. AMK The Strategist

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  5. Dr. AMK

    Dr. AMK The Strategist

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    [​IMG]
    The enormous, 25-meter Giant Magellan Telescope (GMT) will not only usher in a new era in ground-based astronomy, but will take the first cutting-edge images of the Universe where stars are seen exactly as they actually are: without diffraction spikes. (Giant Magellan Telescope — GMTO Corporation)

    World’s Largest Telescope To Finally See Stars Without Artificial Spikes
    https://medium.com/starts-with-a-ba...-stars-without-artificial-spikes-395594271a08
    One of astronomy’s most iconic sights in an artifact of faulty optics. Here’s how a new, great design will overcome it.
    When you look out at the greatest images of the Universe, there are a few sights that light up our memories and fire our imaginations. We can see the planets in our own Solar System to incredible detail, galaxies lying millions or even billions of light years away, nebulae where new stars are being birthed, and stellar remnants that give an eerie, fatalistic look into our cosmic past and our own Solar System’s future. But the most common sight of all are stars, lying everywhere and in any direction we care to look, both in our own Milky Way and beyond. From ground-based telescopes to Hubble, stars almost always come with spikes on them: an image artifact due to how telescopes are constructed. As we prepare for the next generation of telescopes, however, one of them — the 25-meter Giant Magellan Telescope — stands out: it’s the only one that won’t have those artificial spikes.
    [​IMG]
    Hickson compact group 31, as imaged by Hubble, is a spectacular “constellation”, but almost as prominent are the few stars from our own galaxy visible, noted by the diffraction spikes. In only one case, that of the GMT, will those spikes be absent. (ASA, ESA, S. Gallagher (The University of Western Ontario), and J. English (University of Manitoba))
    There are a lot of ways to make a telescope; in principle, all you need to do is collect-and-focus light from the Universe onto a single plane. Early telescopes were built on the concept of a refractor, where the incoming light passes through a large lens, focusing it down to a single point, where it can then be projected onto an eye, a photographic plate, or (in more modern fashion) a digital imaging system. But refractors are limited, fundamentally, by how large you can physically build a lens to the necessary quality. These telescopes barely top 1 meter in diameter, at maximum. Since the quality of what you can see is determined by the diameter of your aperture, both in terms of resolution and light-gathering power, refractors fell out of fashion over 100 years ago.
    [​IMG]
    Reflecting telescopes surpassed refractors long ago, as the size you can build a mirror greatly surpasses the size to which you can build a similar-quality lens. (The Observatories of the Carnegie Institution for Science Collection at the Huntington Library, San Marino, Calif.)
    But a different design — the reflecting telescope — can be far more powerful. With a highly reflective surface, a properly shaped mirror can focus incoming light onto a single point, and mirrors can be created, cast and polished to much greater sizes than lenses can. The largest single-mirror reflectors can be up to a whopping 8-meters in diameter, while segmented mirror designs can go even larger. At present, the segmented Gran Telescopio Canarias, with a 10.4 meter diameter, is the largest in the world, but two (and potentially three) telescopes will break that record in the coming decade: the 25-meter Giant Magellan Telescope (GMT) and the 39-meter Extremely Large Telescope (ELT).


    [​IMG]
    A comparison of the mirror sizes of various existing and proposed telescopes. When GMT comes online, it will be the world’s largest, and will be the first 25 meter+ class optical telescope in history, later to be surpassed by the ELT. But all of these telescopes have mirrors, and each of the ones shown in color (foreground) are reflecting telescopes. (Wikimedia Commons user Cmglee)
    Both of these are reflecting telescopes with many segments, poised to image the Universe like never before. The ELT is larger, is made of more segments, is more expensive, and should be completed a few years after GMT, while the GMT is smaller, made of fewer (but larger) segments, is less expensive, and should reach all of its major milestones first. These include:

    • excavations that began in February of 2018,
    • concrete pouring in 2019,
    • a completed enclosure against weather by 2021,
    • the delivery of the telescope by 2022,
    • the installation of the first primary mirrors by early 2023,
    • first light by the end of 2023,
    • first science in 2024,
    • and a scheduled completion date by the end of 2025.
    That’s soon! But even with that ambitious schedule, there’s one huge optical advantage that GMT has, not only over the ELT, but over all reflactors: it won’t have diffraction spikes on its stars.
    [​IMG]
    The star powering the Bubble Nebula, estimated at approximately 40 times the mass of the Sun. Note how the diffraction spikes, due to the telescope itself, interferes with nearby detailed observations of fainter structures.(NASA, ESA, Hubble Heritage Team)
    These spikes that you’re used to seeing, from observatories like Hubble, come about not from the primary mirror itself, but from the fact that there needs to be another set of reflections that focus the light onto its final destination. When you focus that reflected light, however, you need some way to place-and-support a secondary mirror to refocus that light onto its final destination. There’s simply no way to avoid having supports to hold that secondary mirror, and those supports will get in the way of the light. The number and the arrangement of the supports for the secondary mirror determine the number of spikes — four for Hubble, six for James Webb — you’ll see on all of your images.

    [​IMG]
    Comparison of diffraction spikes for various strut arrangements of a reflecting telescope. The inner circle represents the secondary mirror, while the outer circle represents the primary, with the “spike” pattern shown underneath. (Wikimedia Commons / Cmglee)
    All ground-based reflectors have these diffraction spikes, and so will the ELT. The gaps between the 798 mirrors, despite making up just 1% of the surface area, contribute to the magnitude of the spikes. Whenever you image something faint that unluckily happens to be near something close and bright — like a star — you have these diffraction spikes to contend with. Even by using shear imaging, which takes two almost-identical images that are only slightly mis-positioned and subtract them, you can’t get rid of those spikes entirely.

    [​IMG]
    The Extremely Large Telescope (ELT), with a main mirror 39 metres in diameter, will be the world’s biggest eye on the sky when it becomes operational early in the next decade. This is a detailed preliminary design, showcasing the anatomy of the entire observatory. (ESO)
    But with seven enormous, 8-meter diameter mirrors arranged with one central core and six symmetrically-positioned circles surrounding it, the GMT is brilliantly designed to eliminate these diffraction spikes. These six outer mirrors, the way they’re arranged, allows for six very small, narrow gaps that extend from the edge of the collecting area all the way into the central mirror. There are multiple “spider arms” that hold the secondary mirror in place, but each arm is precisely positioned to run exactly in between those mirror gaps. Because the arms don’t block any of the light that’s used by the outer mirrors, there are no spikes at all.

    [​IMG]
    The 25-meter Giant Magellan Telescope is currently under construction, and will be the greatest new ground-based observatory on Earth. The spidar arms, seen holding the secondary mirror in place, are specially designed so that their line-of-sight falls directly between the narrow gaps in the GMT mirrors. (Giant Magellan Telescope / GMTO Corporation)
    Instead, owing to this unique design — including the gaps between the different mirrors and the spider arms crossing the central primary mirror — there’s a new set of artifacts: a set of circular beads that appear along ring-like paths (known as Airy rings) surrounding each star. These beads will appear as empty spots in the image, and are inevitable based on this design whenever you look. However, these beads are low-amplitude and are only instantaneous; as the sky and the telescope rotate over the course of a night, these beads will be filled in as a long-exposure image is accumulated. After about 15 minutes, a duration that practically every image should attain, those beads will be completely filled in.

    [​IMG]
    The core of the globular cluster Omega Centauri is one of the most crowded regions of old stars. GMT will be able to resolve more of them than ever before, all without diffraction spikes.(NASA/ESA and The Hubble Heritage Team (STScI/AURA))
    The net result is that we’ll have our first world-class telescope that will be able to see stars exactly as they are: with no diffraction spikes around them! There is a slight trade-off in the design to achieve this goal, the biggest of which is that you lose a little bit of light-gathering power. Whereas the end-to-end diameter of the GMT, as designed, is 25.4 meters, you “only” have a collecting area that corresponds to a 22.5 meter diameter. The slight loss of resolution and light-gathering power, however, is more than made up for when you consider what this telescope can do that places it apart from all others.

    [​IMG]
    A selection of some of the most distant galaxies in the observable Universe, from the Hubble Ultra Deep Field. GMT will be capable of imaging all of these galaxies with ten times the resolution of Hubble. (NASA, ESA, and N. Pirzkal (European Space Agency/STScI))
    It will achieve resolutions of between 6–10 milli-arc-seconds, depending on what wavelength you look at: 10 times as good as what Hubble can see, at speeds 100 times as fast. Distant galaxies will be imaged out to distances of ten billion light years, where we can measure their rotation curves, look for signatures of mergers, measure galactic outflows, look for star formation regions and ionization signatures. We can directly image Earth-like exoplanets, including Proxima b, out to somewhere between 15–30 light years distant. Jupiter-like planets will be visible out to more like 300 light years. We’ll also measure the intergalactic medium and the elemental abundances of matter everywhere we look. We’ll find the earliest supermassive black holes.

    [​IMG]
    The more distant a quasar or supermassive black hole is, the more powerful a telescope (and camera) you need to find it. GMT will have the advantage of being able to do spectroscopy on these ultra-distant objects that it finds. (NASA and J. Bahcall (IAS) (L); NASA, A. Martel (JHU), H. Ford (JHU), M. Clampin (STScI), G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the ACS Science Team and ESA (R))
    And we’ll make direct, spectroscopic measurements of individual stars in crowded clusters and environments, probe the substructure of nearby galaxies, and observe close-in binary, trinary and multi-star systems. This even includes stars in the galactic center, located some 25,000 light years away. All, of course, without diffraction spikes.

    [​IMG]
    This image illustrates the improvement in resolution in the central 0.5” of the Galaxy from seeing-limited to Keck + Adaptive Optics to future Extremely Large Telescopes like GMT with adaptive optics. Only with GMT will the stars appear without diffraction spikes. (A. Ghez / UCLA Galactic Center Group — W.M. Keck Observatory Laser Team)
    Compared to what we can presently see with the world’s greatest observatories, the next generation of ground-based telescopes will open up a slew of new frontiers that will peel back the veil of mystery that enshrouds the unseen Universe. In addition to planets, stars, gas, plasma, black holes, galaxies, and nebulae, we’ll be looking for objects and phenomena that we’ve never seen before. Until we look, we have no way of knowing exactly what wonders the Universe has waiting for us. Owing to the clever and innovative design of the Giant Magellan Telescope, however, the objects we’ve missed due to diffraction spikes of bright, nearby stars will suddenly be revealed. There’s a whole new Universe to be observed, and this one, unique telescope will reveal what no one else can see.
     
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  6. Dr. AMK

    Dr. AMK The Strategist

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    [​IMG]
    The most common “sized” world in the galaxy is a super-Earth, between 2 and 10 Earth masses, such as Kepler 452b, illustrated at right. But the illustration of this world as “Earth-like” in any way may be mistaken.(NASA/Ames/JPL-Caltech/T. Pyle)

    Sorry, Super-Earth Fans, There Are Only Three Classes Of Planet
    https://medium.com/starts-with-a-bang/sorry-super-earth-fans-there-are-only-three-classes-of-planet-44f3da47eb64
    And super-Earth, mini-Neptunes, and super-Jupiters aren’t among them. You’ll be surprised to learn why.
    Just 30 years ago, if you had asked an astronomer if there were planets around other stars beyond the Sun, they couldn’t tell you for certain. Although all the theories about planet formation indicated that they should exist around many stars, if not most of them, we had no evidence of planets beyond the Solar System. So we did the most natural thing you can imagine: we assumed others were like our own, with rocky worlds in the inner portions and gas giants in the outer reaches. Over the ensuing decades, we began discovering that our assumptions were gravely mistaken: practically all stars have planets; worlds of all sizes could appear anywhere in a solar system; there were many planets even larger than Jupiter; and most worlds were larger than Earth but smaller than Neptune. Yet, despite all we’ve learned, there seem to be only three classes of planet out there: Terran worlds, Neptunian worlds, and Jovian worlds.
    [​IMG]
    The small Kepler exoplanets known to exist in the habitable zone of their star. Whether these worlds are Earth-like or Neptune-like is an open question, but most of them now appear to be more akin to Neptune than to our own world. (NASA/Ames/JPL-Caltech)
    This is likely not what you’ve heard before, since this isn’t how astronomers have been classifying the planets they’ve been finding. Owing to two major methods:

    1. The radial velocity (or stellar wobble) method, where the mass of a planet is inferred by the periodic orbital motion its star is seen to experience,
    2. and the transit method (used by NASA’s Kepler satellite), where a planet passes in front of a star relative to our line-of-sight, blocking some of its light,
    we’ve been able to measure either the mass or the radius of a large number of planets. Kepler, in particular, is outstanding at measuring an exoplanet’s radius. When we classified them, we found something exciting and surprising: the majority of the planets in the Universe weren’t like the ones in our Solar System.
    [​IMG]
    The numbers of planets discovered by Kepler sorted by their size distribution, as of May 2016, when the largest haul of new exoplanets was released. Super-Earth/mini-Neptune worlds are by far the most common. (NASA Ames / W. Stenzel)
    While rocky, Earth-sized worlds — and slightly larger and slightly smaller rocky worlds — were common, as were Neptune-and-Jupiter sized worlds, there was a third class of planet that was the most common of all. In between the size of Earth and Neptune lied a possibility we had overlooked: a super-Earth (or mini-Neptune) world. As it turned out, there were more super-Earths than any other type. It led many to wonder why our Solar System didn’t have one of these super-Earth-like planets, and whether some catastrophe or rarity happened in our early history to leave us with the results we have today.
    [​IMG]
    While a visual inspection shows a large gap between Earth-size and Neptune-size worlds, the reality is you can only be about 25% larger than Earth and still be rocky. Anything larger, and you’re more of a gas giant. (Lunar and Planetary Institute)
    The possibilities were intriguing but frustrating, including:

    • That early super-Earths formed, but didn’t survive, perhaps getting ejected as the giant planets migrated.
    • That the entire inner Solar System was erased before Jupiter moved outwards, and the rocky worlds are so small because they formed late, after most of the material was gone.
    • Or that our massive gas giants and the Sun gobbled up the early planet-forming material for themselves, eliminating the possibility of a super-Earth.
    But all of this speculation made an important assumption that isn’t necessarily correct: that what we’re calling super-Earths and mini-Neptunes are actually distinct classes of planet from what we have in our Solar System. Is that assumption any good, though?
    [​IMG]
    We’ve classified many worlds outside of our Solar System as being potentially habitable, owing to their distance from their star, their radius and their temperatures. But many of the worlds we’ve found have been classified as ‘super-Earths,’ which look as though they are Neptune-like, rather than Earth-like, having thick hydrogen-and-helium envelopes. (NASA Ames / N. Batalha and W. Stenzel)
    The way you tell is by looking at the data that you have. If you want to be considered a planet, everyone agrees that you need to have enough mass to pull yourself into hydrostatic equilibrium: a sphere if you’re not rotating, a more ellipsoidal shape if you are. We can imagine a lot of different possibilities for these worlds, including:

    • whether they’re rocky or not,
    • whether they have atmospheres or not,
    • whether their surfaces have frozen over or not,
    • whether they have large, hydrogen-and-helium gas envelopes around them,
    • whether their cores are significantly compressed due to gravitation,
    • and whether they’re beginning to fuse light elements into heavier ones inside of them.
    Simple yes-or-no answers to these questions might be relevant not only for the potential habitability of a world, but also for understanding how many types are scientifically reasonable to classify these worlds into.
    [​IMG]
    An illustration of the full suite of planets discovered by Kepler. While the radii shown are accurate, the composition and classification of these worlds has remained speculation, until now. (NASA Ames / W. Stenzel)
    But rather than speculate with the Kepler data, scientists Jingjing Chen and David Kipping came up with a new, intriguing and compelling way of classifying these worlds based on the data alone. By plotting out only the planets that we had measured both the mass and radius of, they were able to identify where there were steady relationships between worlds (indicating similarities), and where there were changes in relationships (indicating changes or transitions). What they found showed us that we’ve been looking at the “problem” all wrong.
    [​IMG]
    The classification scheme of planets as either rocky, Neptune-like, Jupiter-like or stellar-like.(Chen and Kipping, 2016, via https://arxiv.org/pdf/1603.08614v2.pdf)
    As their research (and the above graph) shows, there are only three different types of world that exist! According to their classification schemes, there are:

    1. Terran worlds — these are worlds akin to the rocky worlds in our Solar System. They may have oceans, ices, and/or atmospheres, but don’t have a hydrogen/helium envelope around them.
    2. Neptunian worlds — these are planets akin to Saturn, Uranus and Neptune, and are dominated by a large atmosphere of hydrogen, helium, and other atoms/molecules that are easily boiled-off. They may have rocky interiors, but they obey a different mass/radius relationship than the Terran worlds.
    3. Jovian worlds — akin to Jupiter, these worlds are so massive that they begin to compress on the inside; as you add more mass, their radius shrinks. This effect, of gravitational self-compression, is why Jupiter is only about 20% larger than Saturn, but is three times as massive.
    And that’s it. If you get more massive than that, you’ll start fusing light elements into heavier ones in your core, and become a full-blown star.
    [​IMG]
    Brown dwarfs, between about 13–80 Jupiter masses, will fuse deuterium+deuterium into helium-3 or tritium, remaining at the same approximate size as Jupiter but achieving much greater masses. Note the Sun (in background) is not to scale and would be many times larger.(NASA/JPL-Caltech/UCB)
    Now, there are some extremes likely to be out there, that represent small exceptions to this rule. There are Neptunian or possibly even Jovian worlds that have been so thoroughly blasted by either a star or another astrophysical source that their atmospheres have been stripped away, and all that remains is a rocky, Terran-world-like core. There are Jovian worlds so massive that they start the process of deuterium-burning, becoming a type of failed star known as a brown dwarf. And there may be worlds in the transition zones, either between Terran/Neptunian or Neptunian/Jovian, that maybe have features of both classes of world, depending on various factors like temperature or evolutionary history.
    [​IMG]
    This artist’s impression shows the atmosphere of a Neptune-like planet (foreground) being swept backwards by powerful radiation from an outburst in the center of the Milky Way Galaxy (right). The outburst of X-rays and ultraviolet light is produced by material falling towards the supermassive black hole located there. The planet’s host star is shown on the left. (M. Weiss/CfA)
    What’s really interesting is how the mass/radius relationship changes for these three different classes of world. Up to about double the Earth’s mass, or a size just ~25% larger than Earth’s radius, you have an opportunity to be Earth-like, with thriving life on the surface. Beyond that, you’ll have an enormous hydrogen/helium envelope, and be much more akin to Neptune, Uranus or Saturn. In other words, what we’ve been classifying as “super-Earths” aren’t anything like Earth at all, but are instead gas giant worlds, expected to be wholly inhospitable to life on their surfaces.
    [​IMG]
    A cutaway of Jupiter’s interior. If all the atmospheric layers were stripped away, the core would appear to be a rocky super-Earth, but this illustrates how flawed the ‘super-Earth’ designation actually is.(Kelvinsong/Wikimedia Commons)
    Chen and Kipping come to this very conclusion in their paper, where they answer the question of “where is our Solar System’s super-Earth?” as follows:

    The large number of 2–10 [Earth mass] planets discovered is often cited as evidence that Super-Earths are very common and thus Solar System’s makeup is unusual… However, if the boundary between Terran and Neptunian worlds is shifted down to 2 [Earth masses], the Solar System is no longer unusual. Indeed, by our definition three of the eight Solar System planets are Neptunian worlds, which are the most common type of planet around other Sun-like stars.
    In this classification, the answer becomes clear: Earth-sized is the right size for potential long-term life. Much smaller, and it’s hard to hold onto a rich, life-supporting atmosphere; much larger, and it’s too easy to hold onto a life-crushing hydrogen/helium envelope.
    [​IMG]
    This infographic displays some illustrations and planetary parameters of the seven planets orbiting TRAPPIST-1. They are shown alongside the rocky planets in our Solar System for comparison. A world more than about 25% larger in radius than Earth can no longer be considered to be a Terran-like world, but all of these might actually be rocky. (NASA)
    There are a few planets out there that we’ve discovered so far, such as Kepler-438b, Kepler-186f, Proxima b, and the TRAPPIST-1 worlds, that might have the right combination of mass-and-radius to support life. But most of what we’ve been calling “potentially habitable worlds” out there are simply too large in radius, and hence, with a crushing atmosphere full of volatiles, to be a candidate for life-as-we-know-it in any way at all. There are only three classes of planets out there, the Terran worlds, Neptunian worlds, and Jovian worlds, that make any sort of physical sense. What we’ve been calling super-Earths are just Neptunian worlds that are somewhat smaller than what we find in our Solar System, and they turn out to be the most common type of planet out there. With three Neptunian worlds in our own backyard, we’re not missing anything after all.
     
    Last edited: Mar 10, 2018
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  7. Dr. AMK

    Dr. AMK The Strategist

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    An Astronomer's Guide to The Star Trek Universe : Mapping the United Federation of Planets
     
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  8. Dr. AMK

    Dr. AMK The Strategist

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  9. Dr. AMK

    Dr. AMK The Strategist

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    [​IMG]
    Even though the majority of dark matter in the galaxy exists in a vast halo engulfing us, each individual dark matter particle makes an elliptical orbit under the influence of gravity. If dark matter is its own antiparticle, and we learn how to harness it, it may be the ultimate source of free energy. (ESO / L. Calçada)

    Ask Ethan: Could Dark Matter Not Be A Particle, At All?
    https://medium.com/starts-with-a-ba...-matter-not-be-a-particle-at-all-ba84e99e719d
    We always assume that dark matter is particle-based, and we just need to find which particle it is. But what if it isn’t so?
    Everything we’ve ever detected in the Universe, from matter to radiation, can be broken down into its smallest constituents. Everything in this world is made of atoms, which are made of nuclei and electrons, where nuclei themselves are made of quarks and gluons. Light itself is made of particles: photons. Even gravitational waves, in theory, are made of gravitons: particles we may someday be able to create and detect. But what about dark matter? The indirect evidence for its existence is tremendous and overwhelming, but must it, too, be a particle? That’s what our Patreon supporter Darren Redfern wants to know, as he asks:

    If dark energy can be interpreted as an energy inherent to the fabric of space itself, could it also be possible that what we perceive as “dark matter” is also an inherent function of space itself — either tightly or loosely coupled to dark energy? That is, instead of dark matter being particulate, could it permeate all of space with (homogeneous or heterogeneous) gravitational effects that would explain our observations — more of a “dark mass”?
    Let’s look at the evidence, and see what it tells us about the possibilities.
    [​IMG]
    The expansion (or contraction) of space is a necessary consequence in a Universe that contains masses. But the rate of expansion and how it behaves over time is quantitatively dependent on what’s in your Universe. (NASA / WMAP science team)
    One of the most remarkable features about the Universe is the one-to-one relationship between what’s in the Universe and how the expansion rate changes over time. Through a slew of careful measurements of many disparate sources — including stars, galaxies, supernovae, the cosmic microwave background, and the large-scale structure of the Universe — we’ve been able to measure both of those, determining what our Universe is made of. In principle, there are a slew of various things we can imagine that our Universe might have been made out of, all of which influence cosmic expansion differently.

    [​IMG]
    Various components of and contributors to the Universe’s energy density, and when they might dominate. If cosmic strings or domain walls existed in any appreciable amount, they’d contribute significantly to the expansion of the Universe. (E. Siegel / Beyond The Galaxy)
    Thanks to the full suite of our data, we now know that we’re made of:

    • 68% dark energy, which remains at a constant energy density even as space itself expands,
    • 27% dark matter, which exerts a gravitational force, dilutes as volume increases, and doesn’t measurably interact through any other known force,
    • 4.9% normal matter, which exerts all the forces, dilutes as volume increases, clumps together, and is composed of particles,
    • 0.1% neutrinos, which exert a gravitational and weak force, is made of particles, and clumps together only when they slow down enough to behave as matter instead of radiation,
    • and 0.01% photons, which exert gravitational and electromagnetic forces, act as radiation, and dilutes as both volume increases and its wavelength gets stretched.
    Over time, these various components become relatively more-or-less important, where these percentages represent what the Universe is made out of today.


    [​IMG]
    A plot of the apparent expansion rate (y-axis) vs. distance (x-axis) is consistent with a Universe that expanded faster in the past, but is still expanding today. This is a modern version of, extending thousands of times farther than, Hubble’s original work. The various curves represent Universes made out of different constituent components. (Ned Wright, based on the latest data from Betoule et al. (2014))
    Dark energy, from the best of our measurements, appears to have the same value and properties at every location in space, in all directions on the sky, and at all moments throughout our cosmic history. In other words, dark energy appears both homogeneous and isotropic: it’s the same everywhere and at all times. As well as we know it, dark energy doesn’t need to have a particle; it can easily be a property inherent to the fabric of space itself.

    But dark matter is fundamentally different.


    [​IMG]
    On the largest scales, the way galaxies cluster together observationally (blue and purple) cannot be matched by simulations (red) unless dark matter is included. (Gerard Lemson & the Virgo Consortium, with data from SDSS, 2dFGRS and the Millennium Simulation)
    In order to form the structure that we see in the Universe, particularly on large, cosmic scales, dark matter needs to not only exist, but it needs to clump together. It cannot have the same density at every location in space; rather, it has to be concentrated in overdense regions, and needs to be below-average-density or even completely absent from underdense regions. We can actually tell how much total matter is in a variety of regions of space from a few different sets of observations. What follows are three of the most important.


    [​IMG]
    The large-scale clustering data (dots) and the prediction of a Universe with 85% dark matter and 15% normal matter (solid line) match up incredibly well. The lack of a cutoff indicates the temperature (and coldness) of dark matter; the magnitude of the wiggles indicates the ratio of normal matter to dark matter. (L. Anderson et al. (2012), for the Sloan Digital Sky Survey)
    1.) The Matter Power Spectrum: map out the matter in the Universe, see on what scales galaxies correlate — a measure of the likelihood of finding another galaxy a certain distance away from the one you start with — and plot it out. If you had a Universe that was made of uniform matter, the structure you’d see would be smeared out. If you had a Universe that had dark matter that didn’t clump early on, the structure on the small scales would be destroyed. This matter power spectrum teaches us that approximately 85% of the matter in the Universe is dark matter, totally distinct from protons, neutrons, and electrons, and this dark matter was born cold in temperature, or with a kinetic energy that was small compared to its rest mass.


    [​IMG]
    The mass distribution of cluster Abell 370. reconstructed through gravitational lensing, shows two large, diffuse halos of mass, consistent with dark matter with two merging clusters to create what we see here. Around and through every galaxy, cluster, and massive collection of normal matter exists 5 times as much dark matter, overall. (NASA, ESA, D. Harvey (École Polytechnique Fédérale de Lausanne, Switzerland), R. Massey (Durham University, UK), the Hubble SM4 ERO Team and ST-ECF)
    2.) Gravitational Lensing: take a look at a massive object, like a quasar, galaxy, or cluster of galaxies, and look at how the background light gets distorted by its presence. Because we understand the laws of gravity, as governed by Einstein’s General Relativity, the way that the light bends allows us to infer how much mass is present in each object. Through a slew of other methods, we can determine the amount of mass that’s present in normal matter: stars, gas, dust, black holes, plasma, etc. Again, we find that, on average, 85% of the matter present has to be dark matter, and moreover, that it’s distributed in a more diffuse, cloud-like configuration than the normal matter is. Both weak lensing and strong lensing confirm this.


    [​IMG]
    The structure of the CMB peaks change dependent on what’s in the Universe. (W. Hu and S. Dodelson, Ann.Rev.Astron.Astrophys.40:171–216,2002)
    3.) The Cosmic Microwave Background: if you look at the leftover glow of radiation from the Big Bang, you’ll find that it’s roughly uniform: 2.725 K in all directions. But if you look in more granular detail, you’ll find that there are tiny imperfections on the scales of tens-to-hundreds of µK, on all sorts of angular scales. These fluctuations tell us a slew of important things, including the normal matter/dark matter/dark energy densities, but the biggest thing they tell us is how uniform the Universe was when it was just 0.003% of its current age, and the answer is that the densest region was only about 0.01% denser than the least dense region. In other words, dark matter started out uniform, and then clumped together as time went on!


    [​IMG]
    A detailed look at the Universe reveals that it’s made of matter and not antimatter, that dark matter and dark energy are required, and that we don’t know the origin of any of these mysteries. However, the fluctuations in the CMB, the formation and correlations between large-scale structure, and modern observations of gravitational lensing all point towards the same picture.(Chris Blake and Sam Moorfield)
    Putting all of these together, we come to the conclusion that dark matter must behave like a fluid that permeates the Universe. This fluid has a negligibly small pressure and viscosity, it does respond to radiation pressure, it doesn’t collide with photons or normal matter, it was born cold and non-relativistic, and it clumps together under the force of its own gravity over time. It drives the formation of structure in the Universe on the largest scales. It is highly inhomogeneous, with the magnitude of these inhomogeneities growing over time.



    That’s what we can say about it on large scales, where it’s linked to observation. On small scales, we suspect — but aren’t certain — that this is because dark matter is made up of particles with properties that cause it to behave this way on large scales. The reason we assume this is because the Universe, to the best of our knowledge, is simply composed of particles, end of story! If you’re matter, and if you have mass, you have a quantum counterpart, and that means an indivisible particle at some level. But until we directly detect this particle, there’s no way to rule out the other possibility: that this is some sort of fluidic field that is non-particle-based, but impacts spacetime the same way that an aggregate set of particles would.


    [​IMG]
    Constraints on WIMP dark matter are quite severe, experimentally. The lowest curve rules out WIMP (weakly interacting massive particle) cross-sections and dark matter masses for anything located above it.(Xenon-100 Collaboration (2012), via http://arxiv.org/abs/1207.5988)
    That’s why attempts at direct detection are so important! As a theorist myself who wrote his Ph.D. thesis on large-scale structure formation, I’m well aware that what we can do is incredibly powerful in terms of predicting observables, particularly on large scales. But what we can’t do, theoretically, is confirm whether dark matter is a particle or not. The only way to do that is through direct detection; without it, you can have strong indirect evidence, but it won’t be bulletproof. It doesn’t seem to be coupled to dark energy in any way, since dark energy is truly uniform across space, and the predictions on large scales tell us how it interacts gravitationally and through the other forces quite accurately.


    [​IMG]
    Streams of dark matter drive the clustering of galaxies and the formation of large-scale structure, as shown in this KIPAC/Stanford simulation. (O. Hahn and T. Abel (simulation); Ralf Kaehler (visualization))
    But is it a particle? Until we detect one, we can only assume the answer. The Universe has shown itself to be quantum in nature as far as every other form of matter is concerned, so it’s reasonable to assume dark matter would be as well. Keep in mind, though, that reasoning in this fashion has its limitations. After all, everything follows the same rule everything else follows, but only until they don’t anymore! We’re in uncharted territory with dark matter, and it’s important to be humble before the great unknowns in this Universe.
     
  10. Dr. AMK

    Dr. AMK The Strategist

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    [​IMG]
    The evolution of large-scale structure in the Universe, from an early, uniform state to the clustered Universe we know today. The type and abundance of dark matter would deliver a vastly different Universe if we altered what our Universe possesses. (Angulo et al. 2008, via Durham University)

    Only Dark Matter (And Not Modified Gravity) Can Explain The Universe
    https://medium.com/starts-with-a-ba...gravity-can-explain-the-universe-e272e5f64707
    There have been a lot of public advocates from the “no dark matter” camp, getting lots of popular attention. But the Universe still needs dark matter. Here’s why.
    If you took a look at all the galaxies in the Universe, measured where all the matter you could detect was, and then mapped out how these galaxies were moving, you’d find yourself quite puzzled. Whereas in the Solar System, the planets orbit the Sun with decreasing speed the farther away from the center you go — just as the law of gravitation predicts — the stars around the galactic center do no such thing. Even though the mass is concentrated towards the central bulge and in a plane-like disk, the stars in the outer regions of a galaxy whip around it at the same speeds as they do in the inner regions, defying predictions. Obviously, something is missing. Two solutions spring to mind: either there’s some type of unseen mass out there making up the deficit, or we need to modify the laws of gravity, as we did when we leapt from Newton to Einstein. While both of these possibilities seem reasonable, the unseen mass explanation, known as dark matter, is far and away the superior option. Here’s why.
    [​IMG]
    Individual galaxies could, in principle, be explained by either dark matter or a modification to gravity, but they are not the best evidence we have for what the Universe is made of, or how it got to be the way it is today.(Stefania.deluca of Wikimedia Commons)
    First off, the answer has nothing to do with individual galaxies. Galaxies are some of the messiest objects in the known Universe, and when you’re testing the very nature of the Universe itself, you want the cleanest environment possible. There’s an entire field of study devoted to this, known as physical cosmology. (Full disclosure: it’s my field.) When the Universe was first born, it was very close to uniform: almost exactly the same density everywhere. It’s estimated that the densest region the Universe began with was less than 0.01% denser than the least dense region at the start of the hot Big Bang. Gravitation works very simply and in a very straightforward fashion, even on a cosmic scale, when we’re dealing with small departures from the average density. This is known as the linear regime, and provides a great cosmic test of both gravitation and dark matter.
    [​IMG]
    Large scale projection through the Illustris volume at z=0, centered on the most massive cluster, 15 Mpc/h deep. Shows dark matter density (left) transitioning to gas density (right). The large-scale structure of the Universe cannot be explained without dark matter. (Illustris Collaboration / Illustris Simulation)
    On the other hand, when we’re dealing with large departures from the average, this places you into what’s called the non-linear regime, and these tests are far more difficult to draw conclusions from. Today, a galaxy like the Milky Way may be be a million times denser than the average cosmic density, which places it firmly in the non-linear regime. On the other hand, if we look at the Universe on either very large scales or at very early times, the gravitational effects are much more linear, making this your ideal laboratory. If you want to probe whether modifying gravity or adding the extra ingredient of dark matter is the way to go, you’ll want to look where the effects are clearest, and that’s where the gravitational effects are most easily predicted: in the linear regime.

    Here are the best ways to probe the Universe in that era, and what they tell you.


    [​IMG]
    The fluctuations in the Cosmic Microwave Background were first measured accurately by COBE in the 1990s, then more accurately by WMAP in the 2000s and Planck (above) in the 2010s. This image encodes a huge amount of information about the early Universe, including its composition, age, and history. (ESA and the Planck Collaboration)
    1.) The fluctuations in the Cosmic Microwave Background. This is our earliest true picture of the Universe, and the fluctuations in the energy density at a time just 380,000 years after the Big Bang. The blue regions correspond to overdensities, where matter clumps have begun their inevitable gravitational growth, heading down their path to form stars, galaxies, and galaxy clusters. The red regions are underdense regions, where matter is being lost to the denser regions surrounding it. By looking at these temperature fluctuations and how they correlate — which is to say, on a specific scale. what’s the magnitude of your average fluctuation away from the mean temperature — you can learn an awful lot about the composition of your Universe.


    [​IMG]
    The relative heights and positions of these acoustic peaks, derived from the data in the Cosmic Microwave Background, are definitively consistent with a Universe made of 68% dark energy, 27% dark matter, and 5% normal matter. Deviations are tightly constrained. (Planck 2015 results. XX. Constraints on inflation — Planck Collaboration (Ade, P.A.R. et al.) arXiv:1502.02114)
    In particular, the positions and heights (especially the relative heights) of the seven peaks identified above agree spectacularly with a particular fit: a Universe that’s 68% dark energy, 27% dark matter, and 5% normal matter. If you don’t include dark matter, the relative sizes of the odd-numbered peaks and the even-numbered peaks cannot be made to match up. The best that modified gravity claims can do are to either get you the first two peaks (but not the third or beyond), or to get you the right spectrum of peaks by also adding some dark matter, which defeats the whole purpose. There are no known modifications to Einstein’s gravity that can reproduce these predictions, even after-the-fact, without also adding dark matter.


    [​IMG]
    An illustration of clustering patterns due to Baryon Acoustic Oscillations, where the likelihood of finding a galaxy at a certain distance from any other galaxy is governed by the relationship between dark matter and normal matter. As the Universe expands, this characteristic distance expands as well, allowing us to measure the Hubble constant. (Zosia Rostomian)
    2.) The large-scale structure in the Universe. If you have a galaxy, how likely are you to find another galaxy a certain distance away? And if you look at the Universe on a certain volumetric scale, what departures from the “average” numbers of galaxies do you expect to see there? These questions are at the heart of understanding large-scale structure, and their answers depend very strongly on both the laws of gravity and what’s in your Universe. In a Universe where 100% of your matter is normal matter, you’ll have big suppressions of structure formation on specific, large scales, while if your Universe is dominated by dark matter, you’ll get only small suppressions superimposed on a smooth background. You don’t need any simulations or nonlinear effects to probe this; this can all be calculated by hand.


    [​IMG]
    The data points from our observed galaxies (red points) and the predictions from a cosmology with dark matter (black line) line up incredibly well. The blue lines, with and without modifications to gravity, cannot reproduce this observation without dark matter.(S. Dodelson, from http://arxiv.org/abs/1112.1320)
    When we look out at the Universe on these largest scales, and compare with the predictions of these different scenarios, the results are incontrovertible. Those red points (with error bars, as shown) are the observations — the data — from our own Universe. The black line is the prediction of our standard ΛCDM cosmology, with normal matter, dark matter (in six times the amount of normal matter), dark energy, and general relativity as the law governing it. Note the small wiggles in it and how well — how amazingly well — the predictions match up to the data. The blue lines are the predictions of normal matter with no dark matter, in both standard (solid) and modified gravity (dotted) scenarios. And again, there are no modifications to gravity that are known that can reproduce these results, even after-the-fact, without also including dark matter.


    [​IMG]
    The pathway that protons and neutrons take in the early Universe to form the lightest elements and isotopes: deuterium, helium-3, and helium-4. The nucleon-to-photon ratio determines how much of these elements we will wind up with in our Universe today. These measurements allow us to know the density of normal matter in the entire Universe very precisely. (E. Siegel / Beyond The Galaxy)
    3.) The relative abundance of light elements formed in the early Universe. This isn’t specifically a dark matter-related question, nor is it extremely dependent on gravity. But due to the physics of the early Universe, where atomic nuclei are blasted apart under high-enough energy conditions when the Universe is extremely uniform, we can predict exactly how much hydrogen, deuterium, helium-3, helium-4, and lithium-7 should be left over from the Big Bang in the primordial gas we see today. There’s only one parameter that all of these results depend on: the ratio of photons to baryons (protons and neutrons combined) in the Universe. We’ve measured the number of photons in the Universe thanks to both the WMAP and Planck satellites, and we’ve also measured the abundances of those elements.


    [​IMG]
    The predicted abundances of helium-4, deuterium, helium-3 and lithium-7 as predicted by Big Bang Nucleosynthesis, with observations shown in the red circles. (NASA / WMAP Science Team)
    Putting that together, they tell us the total amount of normal matter in the Universe: it’s 4.9% of the critical density. In other words, we know the total amount of normal matter in the Universe. Its a number that’s in spectacular agreement with both the cosmic microwave background data and the large-scale structure data, and yet, it’s only about 15% of the total amount of matter that has to be present. There is, again, no known modification of gravity that can give you those large-scale predictions and also give you this low abundance of normal matter.


    [​IMG]
    Cluster MACS J0416.1–2403 in the optical, one of the Hubble Frontier Fields that reveals, through gravitational lensing, some of the deepest, faintest galaxies ever seen in the Universe. (NASA / STScI)
    4.) The gravitational bending of starlight from large cluster masses in the Universe. When we look at the largest clumps of mass in the Universe, the ones that are closest to still being in the linear regime of structure formation, we notice that the background light from them is distorted. This is due to the gravitational bending of starlight in relativity known as gravitational lensing. When we use these observations to determine what the total amount of mass present in the Universe is, we get that same number we’ve gotten all along: about 30% of the Universe’s total energy must be present in all forms of matter, added together, to reproduce these results. With only 4.9% present in normal matter, this implies there must be some sort of dark matter present.


    [​IMG]
    Gravitational lensing in galaxy cluster Abell S1063, showcasing the bending of starlight by the presence of matter and energy. (NASA, ESA, and J. Lotz (STScI))
    When you look at the full suite of data, rather than just some small details of what goes on in the messy, complex, nonlinear regime, there’s no way to obtain the Universe we have today without adding in dark matter. People who use Occam’s Razor (incorrectly) to argue in favor of MOND, or MOdified Newtonian Dynamics, need to consider that modifying Newton’s law will not solve these problems for you. If you use Newton, you miss out on the successes of Einstein’s relativity, which are too numerous to list here. There’s the Shapiro time delay. There’s gravitational time dilation and gravitational redshift. There’s the framework of the Big Bang and the concept of the expanding Universe. There’s the Lens-Thirring effect. There are the direct detections of gravitational waves, with their measured speed equal to the speed of light. And there are the motions of galaxies within clusters and of the clustering of galaxies themselves on the largest scales.


    [​IMG]
    On the largest scales, the way galaxies cluster together observationally (blue and purple) cannot be matched by simulations (red) unless dark matter is included. (Gerard Lemson & the Virgo Consortium, with data from SDSS, 2dFGRS and the Millennium Simulation)
    And for all of these observations, there is no single modification of gravity that can reproduce these successes. There are a few vocal individuals in the public sphere who advocate for MOND (or other modified gravity incarnations) as a legitimate alternative to dark matter, but it simply isn’t one at this point. The cosmology community isn’t dogmatic at all about the need for dark matter; we “believe in” it because all of these observations demand it. Yet despite all the efforts going into modifying relativity, there are no known modifications that can explain even two of these four points, much less all four. But dark matter can, and does.

    Just because dark matter appears to be a fudge factor to some, compared to the idea of modifying Einstein’s gravity, doesn’t give the latter any additional weight. As Umberto Eco wrote in Foucault’s Pendulum, “As the man said, for every complex problem there’s a simple solution, and it’s wrong.” If someone tries to sell you modified gravity, ask them about the cosmic microwave background. Ask them about large-scale structure. Ask them about Big Bang Nucleosynthesis and the full suite of other cosmological observations. Until they have a robust answer that’s as good as dark matter’s, don’t let yourself be satisfied.


    [​IMG]
    Four colliding galaxy clusters, showing the separation between X-rays (pink) and gravitation (blue), indicative of dark matter. On large scales, cold dark matter is necessary, and no alternative or substitute will do.(X-ray: NASA/CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A. Mahdavi et al. (top left); X-ray: NASA/CXC/UCDavis/W.Dawson et al.; Optical: NASA/ STScI/UCDavis/ W.Dawson et al. (top right); ESA/XMM-Newton/F. Gastaldello (INAF/ IASF, Milano, Italy)/CFHTLS (bottom left); X-ray: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University) (bottom right))
    Modified gravity cannot successfully predict the large-scale structure of the Universe the way that a Universe full of dark matter can. Period. And until it can, it’s not worth paying any mind to as a serious competitor. You cannot ignore physical cosmology in your attempts to decipher the cosmos, and the predictions of large-scale structure, the microwave background, the light elements, and the bending of starlight are some of the most basic and important predictions that come out of physical cosmology. MOND does have a big victory over dark matter: it explains the rotation curves of galaxies better than dark matter ever has, including all the way up to the present day. But it is not yet a physical theory, and it is not consistent with the full suite of observations we have at our disposal. Until that day comes, dark matter will deservedly be the leading theory of what makes up the mass in our Universe.
     
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