Why is the CMB not simply travelling parallel to us?

Why is the CMB not simply travelling parallel to us?

When we look to the distant farthest reaches of the universe we see light that was emitted at the big bang 14 billion years ago. But the universe was tiny back then so that light, which is only reaching us now, was emitted pretty much right next to us (in galactic terms).

So it's been traveling for 14 billion years at the speed of light and is only reaching us right now, despite only having set off from right next door.

The standard explanation of how it's covered such a tiny distance begins with "expansion", which means the light has been swimming against the tide effectively and since that doesn't fit properly, it's augmented by inventing inflation which is basically a huge rush of Hubbly expansion at the outset, to increase the distance away from us before it began swimming towards us in accordance with Hubble's law which we now observe.

If we instead explain how it's covered such a tiny distance by saying that we are all departing the big bang at or near the speed of light because we are actually for example something like Hawking Radiation, then we don't need "Expansion" do we? Because the CMB is just the light that was emitted near parallel to us and has been traveling through space next to us all this time. So it's quite natural that it is only now reaching us. And then we don't need to invent inflation, do we, to make Hubble's law and the Big Bang fit?

And since it's been traveling so close to parallel all this time it seems natural that it should have such a low level of energy - unlike light emitted right next to us recently.

And then we have a natural reason for mass-energy equivalence don't we, because everything's moving at the speed of light anyway.

And then we have a natural reason for quantum theory don't we because if everything around us is moving nearly parallel at the speed of light then only its direction can change, not its speed so we would expect to find that if we impose a speed vector on objects which only have direction, their degrees of freedom will be over-specified and we won't be able to define their parameters all at once because they don't really exist.

And then it becomes obvious that "perceived speed" is simply an aggregation of how parallel the component parts of something are, so simple objects containing minimal component parts must travel at the speed light while compound things can move more slowly because they have component parts moving in different, changing directions around each other and their overall motion is the combination of the parts.

And then that would predict that a photon can't age.

And then everything seems to make a lot more sense because when we look out at the "Big Bang" in distant space and see it receding at the speed of light, well that's what we know it to be doing anyway so whoopdedoo who cares.

And because the universe is now just a set of paths in 4-dimensional space with no "speed", it should be described by a rotation group SO(3,1) and oh hang on haven't I seen that somewhere before, like the set of transformations in relativity?

But then we would all have to be made of light. So how do we design an experiment in which we bounce beams of appropriately formed light off each other itself at exceptionally low angles of incidence in order to prove that they do, to the surprise of almost the entire physics community, bounce off each other?

And how did we get in such a pickle inventing all this inflation and dark matter?

Some of your questions seem to be genuine enough, so I'll address those:

When the CMB was emitted, it was emitted everywhere in the Universe, in all directions. That means that we see it at all times, but the part that we see at a given time comes from an ever-increasing distance. Today that distance is roughly 47 billion lightyears (Gly). It is also redshifted, today by a factor of $z simeq 1100$. That means that when it was emitted, the Universe was a factor 1100 smaller (in all three directions). Hence, that particular part of the Universe that emitted the light that we see today was $47,mathrm{Gly} ,/, 1100 simeq 43 ,mathrm{Mly}$.

Since the CMB is observed from all directions, only light from small patches on the sky can be regarded as being parallel. Obviously light from the left of you and above you is not parallel.

The CMB hasn't really been "right on our heels" since it was emitted, any more than, say, the cars overtaking you when you riding your bike are right on your heels. You're just constantly being overtaken by new CMB photons. The part we see today was actually carried away from us by expansion for the first 4 billion years or so, before it overtook expansion and started reducing its distance from us.

Inflation, as James Kilfiger comments, is something completely unrelated, taking place when the Universe was some $10^{-33}$ s old, as opposed to the CMB which was emitted when the Universe was 380,000 years old.

Creation argument against the big bang no longer sustainable&mdashCMB shadows and galaxy clusters

I have previously made the argument that the cosmic microwave background (CMB) radiation, &lsquolight&rsquo allegedly from the big bang fireball, casts no shadows in the foreground of galaxy clusters. 1 If the big bang were true, the light from the fireball should cast a shadow in the foreground of all galaxy clusters. This is because the source of the CMB radiation, in standard big bang cosmology, is what is known as the &ldquolast scattering surface&rdquo. 2

The last scattering surface is the stage of the big bang fireball that describes the situation when big bang photons cooled to about 1,100 K. At that stage of the story those photons separated from the plasma that had previously kept them bound. Then expansion of the universe is alleged to have further cooled those photons to about 3 K, which brings them into the microwave band. Thus if these CMB photons cast no shadows in front of all galaxy clusters it spells bad news for the big bang hypothesis.

Did Scientists Actually Spot Evidence Of Another Universe?

The detailed, all-sky picture of the infant universe created from nine years of WMAP data. The image . [+] reveals 13.77 billion year old temperature fluctuations (shown as color differences) that correspond to the seeds that grew to become the galaxies. The signal from our galaxy was subtracted using the multi-frequency data. This image shows a temperature range of ± 200 microKelvin.

Credit: NASA/WMAP Science Team

In a study published earlier this month, a team of theoretical physicists is claiming to have discovered the remnants of previous universes hidden within the leftover radiation from the Big Bang. Our universe is a vast collection of observable matter, like gas, dust, stars, etc., in addition to the ever-elusive dark matter and dark energy. In some sense, this universe is all we know, and even then, we can only directly study about 5% of it, leaving 95% a mystery that scientists are actively working to solve. However, this group of physicists is arguing that our universe isn't alone it's just one in a long line of universes that are born, grow, and die. Among these scientists is mathematical physicist Roger Penrose, who worked closely with Stephen Hawking and currently is the Emeritus Rouse Ball Professor of Mathematics at Oxford University. Penrose and his collaborators follow a cosmological theory called conformal cyclic cosmology (CCC) in which universes, much like human beings, come into existence, expand, and then perish.

OXFORD, ENGLAND - MARCH 22: Professor Sir Roger Penrose, physicist, mathematician and cosmologist, . [+] on Day 2 of the FT Weekend Oxford Literary Festival on March 22, 2015 in Oxford, England. (Photo by David Levenson/Getty Images)

As a universe ages, it expands, and the constituent parts grow farther and farther apart from each other. Consequently, the interactions between galaxies that drive star formation and evolution become rarer. Eventually, the stars die out, and the remaining gas and dust is captured by black holes. In one of his most famous theories, Stephen Hawking proposed that this isn't the end black holes might have a way to slowly lose mass and energy by radiating certain particles. So, after many eons, the remaining black holes in the universe would disappear, leaving only disparate particles. Seemingly a wasteland, this end-state eventually mirrors the environment of our universe's birth, and so, the cycle starts anew.

Artist's logarithmic scale conception of the observable universe with the Solar System at the . [+] center, inner and outer planets, Kuiper belt, Oort cloud, Alpha Centauri, Perseus Arm, Milky Way galaxy, Andromeda galaxy, nearby galaxies, Cosmic Web, Cosmic microwave radiation and Big Bang's invisible plasma on the edge.

Credit: Wikipedia/Pablo Carlos Budassi

When our universe was very young, before any recognizable components like stars, planets, or galaxies formed, it was filled with a dense, hot soup of plasma. As the universe expanded, it cooled, and eventually, particles could combine to form atoms. Eventually, the interaction and fusion of these atoms resulted in all of the matter that we observe today. However, we can still observe the leftover radiation from that initial, dense period in our universe's history. This leftover glow, called the Cosmic Microwave Background (CMB), is the oldest electromagnetic radiation, and it fills the entirety of our universe. If the CCC theory were true, then there would be hints of previous universes in our universe's CMB.

At the end of a universe, when those final black holes dissolve, CCC theory states they should leave behind a signature that would survive the death of that universe and persist into the next. Although not definitive proof of previous universes, detecting that signature would be strong evidence in support of CCC theory. In searching for these "Hawking points", cosmologists face a difficult obstacle as the CMB is faint and varies randomly. However, Penrose is claiming that a comparison between a model CMB with Hawking points and actual data from our CMB has proven that Hawking points actually exist. If true, this would be the first-ever detection of evidence from another universe.

Image of the CMB with Hawking points highlighted.

Credits: Daniel An, Krzysztof A. Meissner and Roger Penrose, BICEP2 Collaboration, V. G. Gurzadyan

Unfortunately, as groundbreaking as this discovery seems, the scientific community has largely dismissed it. One of the fundamental characteristics of the CMB is that, although it has patterns, the variations are entirely statistically random. In fact, Penrose's former collaborator, Stephen Hawking, spotted his own initials in the CMB while others have found a deer, a parrot, and numerous other recognizable shapes in the noise. Similarly, the Wilkinson Anisotropy Microscope Probe that mapped the CMB released an interactive image where you can search for familiar shapes and patterns. An avoidable result of both these random fluctuations and the sheer size of the CMB is that if scientists look hard enough, they can find whatever pattern they need, like the existence of Hawking points, perhaps. Another criticism of Penrose's claim is that if CCC theory holds true, our universe should have tens of thousands of Hawking points in the CMB. Regrettably, Penrose could find only about 20.

“SH” initials of Stephen Hawking are shown in the ILC sky map. The “S” and “H” are in roughly the . [+] same font size and style, and both letters are aligned neatly along a line of fixed Galactic latitude.

Credit: Bennett et al (2011), published in the Astrophysical Journal, 2011.

Still, the possibility of alternate universes, whether long-dead or existing in parallel to our own, is tantalizing. Many other theories also claim to find traces of other universes hiding in the patterns of the CMB as well. Although it sounds like science fiction, we are left to wonder: is this just the cosmological equivalent of seeing shapes in random clouds or will scientists one day discover that we are one among many infinite universes?

The Multiverse

Instead of a massive void, an idea made popular by astrophysicist Laura Mersini-Houghton, posits the possibility that the CMB cold spot is the result of a parallel universe that bumped into our own at some point in the past – a concept that many quantum physicists have entertained, but one that is at odds with traditional physics.

But Mersini-Houghton is no stranger to contentious theories. Just a few years ago she sparked controversy in the scientific world when she claimed that black holes do not exist – an idea that contradicted the work of notable astrophysicists like the late Stephen Hawking.

The multiverse, however, is a concept that has a lot of backing by string theorists, including Dr. Michio Kaku, who believes that everything is connected throughout a multitude of universes via vibrating cosmic strings. String theory states that our universe is just one of 10 500 possible universes.

Mersini-Houghton is a proponent of the multiverse theory and says she believes the CMB cold spot to be the product of quantum entanglement between our universe and another, before they were separated by cosmic inflation. Einstein referred to this quantum behavior as spooky action at a distant.

String theory is complex, but the basic premise in terms of the multiverse is that the Big Bang was the result of an interaction between multiple universes. With the multiverse theory, a universe can grow new universes or split into two separate universes. This was first proposed by Alan Guth as part of his inflation theory.

Inflation theory is based on the counterintuitive idea that gravity can act as a repulsive force, rather than a purely attractive force. This has been hypothesized in the form of a white hole, the opposite of a black hole. Einstein’s theory of relativity allows for the possibility of white holes, connected to a black hole via a wormhole.

When a black hole pulls in everything around it, it becomes increasingly more dense. The light and matter it sucks in is then transported through a wormhole, before being ejected through a white hole. Kaku and Guth say they believe it’s possible that a white hole shooting out matter from anther universe could have resulted in the Big Bang that created our universe.

Are Parallel Universes Real? Here Are Physicists' Leading Multiverse Theories

Right now there might be a whole other universe where instead of brown hair you have red hair, or a universe where you're a classical pianist, not an engineer. In fact, an infinite number of versions of you may exist in an infinite number of other universes.

The idea sounds like science fiction, but multiverse theories — especially those that are actually testable — are gaining traction among physicists. Here are three of the most compelling theories:

If the universe is infinite, multiple universes probably exist.

If the universe is infinite, like many believe it is, then there must be huge patches of the universe that are simply too distant for us to see.

Our own universe is defined by the sphere-shaped amount of light that has had time to reach us. The universe is 13.8 billion years old, so any patches more than 13.8 billion light-years away aren't visible to us. In that sense, multiple universes exist outside our own visible universe simply because the light from them hasn't had enough time to reach us.

The implications are mind-bending.

"If the universe is truly infinite, if you travel outwards from Earth, eventually you will reach a place where there's a duplicate cubic meter of space," Fraser Cain explained to Universe Today. "The further you go, the more duplicates you'll find."

That means there could be another you out there in the universe — or an infinite number of yous.

Scientists are trying to figure out if the universe is finite or infinite by studying signatures in the cosmic microwave background, or the radiation left over from the Big Bang. But the bottom line, according to physicist Joseph Silk, is, "we may never know."

The Big Bang and inflation suggest the existence of a vast multiverse.

The Big Bang theory suggests that when the universe was just a fraction of a second old, it underwent a period of rapid inflation where it "expanded faster than the speed of light," according to Expansion then slowed down, but there's lots of evidence that it kept happening and is still happening.

Some physicists think parts of space-time may have expanded faster than others after the Big Bang, creating "bubble" universes.

So if inflation is real, our universe might just be a bubble floating in a whole bubble bath of other sphere-shaped universes.

Inflation is carrying them farther and farther away from us, so we'd have to invent faster-than-light travel if we ever wanted to visit one.

"It's hard to build models of inflation that don't lead to a multiverse," Alan Guth, a theoretical physicist from Massachusetts Institute of Technology, said in 2014. "It's not impossible, so I think there's still certainly research that needs to be done. But most models of inflation do lead to a multiverse, and evidence for inflation will be pushing us in the direction of taking [the idea of a] multiverse seriously."

Some physicists think it's possible to prove the bubble idea. When our own bubble universe was first forming, it may have collided with another bubble universe before inflation separated us.

Physicists like Matthew Johnson are searching through the cosmic microwave background (radiation leftover after the Big Bang) for signs of collisions. Johnson thinks the collisions might have left behind visible bruises:

Gravity is hiding in other universes

Physicists have no idea why gravity is so much weaker than the other fundamental forces. Some theories imply the existence of parallel universes.

"A small fridge magnet is enough to create an electromagnetic force greater than the gravitational pull exerted by planet Earth," the European Organization for Nuclear Research, better known as CERN, explains. "One possibility is that we don't feel the full effect of gravity because part of it spreads to extra dimensions. Though it may sound like science fiction, if extra dimensions exist, they could explain why the universe is expanding faster than expected, and why gravity is weaker than the other forces of nature."

Physicists are actually searching for evidence of other dimensions inside the Large Hadron Collider at CERN. Entire universes full of red-headed, piano-playing yous could be hiding inside those extra dimensions.

Signs of a Multiverse? Scientists May Have Found Evidence We Bumped Into a Parallel Universe

Scientists have long tried to explain the origin of a mysterious, large and anomalously cold region of the sky. In 2015, they came close to figuring it out as a study showed it to be a "supervoid" in which the density of galaxies is much lower than it is in the rest of the universe. However, other studies haven't managed to replicate the result.

Now, new research led by Durham University, submitted for publication in the Monthly Notices of the Royal Astronomical Society, suggests the supervoid theory doesn't hold up. Intriguingly, that leaves open a pretty wild possibility&mdashthe cold spot might be the evidence of a collision with a parallel universe. But before you get too excited, let's look at how likely that would actually be.

The cold spot can be seen in maps of the "cosmic microwave background" (CMB), which is the radiation left over from the birth of the universe. The CMB is like a photograph of what the universe looked like when it was 380,000 years old and had a temperature of 3,000 degrees Kelvin (2,700 degrees Celsius). What we find is that it is very smooth with temperature deviations of less than one part in 10,000. These deviations can be explained pretty well by our models of how the hot universe evolved up to an age of 380,000 years.

However the cold spot is harder to work out. It is an area of the sky about five degrees across that is colder by one part in 18,000. This is readily expected for some areas covering about one degree&mdashbut not five. The CMB should look much smoother on such large scales.

The power of galaxy data


The GAMA survey, and other surveys like it, take the "spectra" of thousands of galaxies. Spectra are images of light captured from a galaxy and spread out according to its wavelengths. This provides a pattern of lines emitted by the different elements in the galaxy. The further away the galaxy is, the more the expansion of the universe shifts these lines to appear at longer wavelengths than they would appear on Earth. The size of this so-called "redshift" therefore gives the distance to the galaxy. Spectra coupled with positions on the sky can give us 3D maps of galaxy distributions.

But the researchers concluded that there simply isn't a large enough void of galaxies to explain the cold spot &ndash there was nothing too special about the galaxy distribution in front of the cold spot compared to elsewhere.


Controversial interpretation

The idea that we live in a "multiverse" made up of an infinite number of parallel universes has long been considered a possibility. But physicists still disagree about whether it could represent a physical reality or whether it's just a mathematical quirk. It is a consequence of important theories like quantum mechanics, string theory and inflation.

Quantum mechanics oddly states that any particle can exist in "superposition"&mdashwhich means it can be in many different states simultaneously (such as locations). This sounds bizarre but it has been observed in laboratories. For example, electrons can travel through two slits at the same time&mdashwhen we are not watching. But the minute we observe each slit to catch this behaviour, the particle chooses just one. That is why, in the famous "Shroedinger's cat" thought experiment, an animal can be alive and dead at the same time.



There is no particular reason why we should just now be seeing the imprint of a colliding universe. From what we know about how the universe formed so far, it seems likely that it is much larger than what we can observe. So even if there are parallel universes and we had collided with one of them&mdashunlikely in itself&mdashthe chances that we'd be able to see it in the part of the universe that we happen to be able to observe on the sky are staggeringly small.


It seems that the cold spot in the sky will continue to be a mystery for some time. Although many of the explanations out there seem unlikely, we don't necessarily have to dismiss them as pure fantasy. And even if it takes time to find out, we should still revel in how far cosmology has come in the last 20 years. There's now a detailed theory explaining, for the most part, the glorious temperature maps of the CMB and the cosmic web of galaxies which span across billions of light years.

Answers and Replies

If inflation didnt occur. The rest still occured like having Cosmic Microwave Background, right?

The last two paragraph about Inflation at Wiki:

"In order to work, and as pointed out by Roger Penrose from 1986 on, inflation requires extremely specific initial conditions of its own, so that the problem (or pseudo-problem) of initial conditions is not solved: "There is something fundamentally misconceived about trying to explain the uniformity of the early universe as resulting from a thermalization process. [. ] For, if the thermalization is actually doing anything [. ] then it represents a definite increasing of the entropy. Thus, the universe would have been even more special before the thermalization than after."[132] The problem of specific or "fine-tuned" initial conditions would not have been solved it would have gotten worse. At a conference in 2015, Penrose said that "inflation isn't falsifiable, it's falsified. [. ] BICEP did a wonderful service by bringing all the Inflation-ists out of their shell, and giving them a black eye."[7]

A recurrent criticism of inflation is that the invoked inflaton field does not correspond to any known physical field, and that its potential energy curve seems to be an ad hoc contrivance to accommodate almost any data obtainable. Paul Steinhardt, one of the founding fathers of inflationary cosmology, has recently become one of its sharpest critics. He calls 'bad inflation' a period of accelerated expansion whose outcome conflicts with observations, and 'good inflation' one compatible with them: "Not only is bad inflation more likely than good inflation, but no inflation is more likely than either [. ] Roger Penrose considered all the possible configurations of the inflaton and gravitational fields. Some of these configurations lead to inflation [. ] Other configurations lead to a uniform, flat universe directly – without inflation. Obtaining a flat universe is unlikely overall. Penrose's shocking conclusion, though, was that obtaining a flat universe without inflation is much more likely than with inflation – by a factor of 10 to the googol (10 to the 100) power!"[5][114] Together with Anna Ijjas and Abraham Loeb, he wrote articles claiming that the inflationary paradigm is in trouble in view of the data from the Planck satellite.[133][134] Counter-arguments were presented by Alan Guth, David Kaiser, and Yasunori Nomura[135] and by Andrei Linde,[136] saying that "cosmic inflation is on a stronger footing than ever before".[135]"

A ɼold Spot' Detected In Space Could Confirm The Existence Of Parallel Universes, According To New Findings

Main Image: NASA







Richard Dawkins.Net


University of Durham


ESA Planck Collaboration




The big bang isn’t the only model dealing with the origin of the universe. One of the more fascinating theories centers around an anomaly found in the CMBR. The theory – called “ conformal cyclic cosmology “ – was developed by Sir Roger Penrose, a physicist from the University of Oxford. He believes that our universe is not the first, nor will it be the last, universe to crop up from a big bang.

“Conformal cyclic cosmology” suggests that our big bang theory is far too incomplete to be viable. It doesn’t offer an explanation as to why the initial conditions of the universe were geared toward a low-entropy, highly ordered state. That is, UNLESS things were set in motion prior to the “bang” in the big bang. Moreover, when our universe starts to head toward its own end, it will divert back into the universe it was originally, before starting anew. We can thank the general nature of black holes for this, but

Well.. As you’ve all probably heard someone say, black holes are the vacuum cleaners of the universe (a claim that is a misnomer, by the way) they consume obects that come to close and gradually rid the universe of entropy. Yet they, like all things, eventually meet their end. A lot of things happen in the window of time separating the end from the beginning, like collisions.



When black holes come too close and crash into each other, they release gravitational waves a phenomenon that sees spacetime become distorted like ripples in a pond. It has been theorized that the big bang itself should have produced gravitational waves (an assertion that was recently confirmed and then challenged soon after ), which could potentially be picked up in the CMBR data, opening the door for us to witness the aftermath if only we knew what to look for. Like, in this case, curling in the polarization of the light in the CMB.

With all of that in mind: as we said before, all temperature variations in the CMB data would be completely random. In yet another example of the opposite being found, Penrose — along his partner Vahe Gurzadyan, who hails from the Yerevan Physics Institute in Armenia — have found several concentric circles in the CMB — places where temperature variations formulate patterns stashed among the warmer regions. They believe these circles are indicative of black hole collisions in the universe that collapsed to form our own (so they are effectively giving us information about the universe before the big bang ).

If the men are right, someday in the far, far future — when the universe will no longer be able to expand any further — entropy will digress back to its original state, before the universe collapses in on itself and rebounds, ushering in a new big bang. When that happens, the gravitational waves from our universe might be recorded in the new CMB, allowing a new epoch of aliens to ponder the same questions we currently have no answers to.

Yoga for dark matter: Making the Cold Dark Matter model more flexible

This guest post was written by Tanvi Karwal. Tanvi is a grad student at Johns Hopkins studying cosmology. She’s a theorist who likes hunting for evidence of exotic kinds of dark matter and dark energy in the cosmic microwave background. She has also dabbled in cosmic rays. In her free time, she draws puns on the blackboards at work when money is short and scuba dives when it is not.

Authors: Wayne Hu

First Author’s Institution: Princeton University

Status: Published by The Astrophysical Journal, open access

Why should dark matter do yoga?

The CDM (-Cold Dark Matter) model is the current standard model of cosmology and consists of normal matter, photons, neutrinos, cold dark matter and dark energy occupying a flat universe. The aptly-named Dark Sector can be described very simply: dark energy is a Cosmological Constant () and dark matter is cold (i.e. its particles move slowly compared to the speed of light). Each component of CDM can be quantified by a single parameter. Dark energy drives cosmic acceleration, while dark matter clusters and seeds structure formation.

The simple CDM model has done exceptionally well in describing the large-scale structure of the universe: it accurately predicts features in phenomena such as baryon acoustic oscillations, the matter power spectrum and the cosmic microwave background (CMB). Even in the local universe, measurements of dark energy do not deviate signi ficantly from  and CDM has had successes explaining galaxy clusters.

Thankfully, CDM might not be the whole picture (or I would be out of a job!). When a CDM model is fit to the CMB, the model under-predicts the current rate of expansion of the universe (the infamous Hubble tension). Physically motivating a cosmological-constant-like dark energy has left physicists bending over backwards (even when they’re not doing yoga). Dark matter has not yet been directly detected and models of dark matter remain limited to candidates that are considered to be well-motivated theoretically. Hence, the Dark Sector remains, well, dark – despite some recent progress, our understanding is woefully incomplete.

If CDM is not the whole picture, how can we construct a more complete model? One way to assemble the pieces of this cosmological puzzle is to employ a phenomenological approach, rather than deriving the model from first principles. A phenomenological model describes relationships between variables based on experiment and observations, instead of being derived from fundamental theory.

Making dark matter do yoga with a Generalised Dark Matter model

The Generalised Dark Matter (GDM) model, first proposed by Wayne Hu in 1998, is precisely such a phenomenological approach. GDM introduces flexibility into the Dark Sector that is not possible in CDM.

GDM postulates that dark matter is really like an imperfect fluid. In this description, dark matter has a non-zero intrinsic pressure, given by times its energy density, where is a dimensionless equation-of-state parameter. Because dark matter has now been imbued with a non-zero pressure, it can have pressure waves with some e ffective sound speed (just like the speed of sound [link to video] in air is 343 m/s). Additionally, it has a non-zero viscosity that damps the pressure waves.

As the GDM model incorporates pressure, unlike its cold counterpart, we can observe some interesting features from this property alone. As the energy density of any species of particle is proportional to the scale factor , its energy density evolves di fferently relative to CDM. If GDM replaces CDM, the energy density of the dominant matter species can evolve di fferently over time compared to CDM. Replacing with can alter the time at which the universe exhibited matter-radiation equality, the time of the emission of the CMB and therefore changes the features in the CMB itself.

And that’s not all. If GDM has pressure waves, its energy density can oscillate about a mean value. If it is viscous, these oscillations will be damped and decay. GDM therefore changes how matter clusters and hence the shape of the matter power spectrum.

Furthermore, clustered matter forms a gravitational potential well. As GDM continues to oscillate, so does the depth of these wells. CMB photons travelling toward us through the cosmos gain and lose energy to these wells through a mechanism known as the Integrated Sachs-Wolfe (ISW) e ffect. Photons gain energy by being attracted into gravitational potential wells, and lose energy as they try to climb back out of the wells. If the height of the well changes as the photon is travelling through it, it emerges from the well with a little less or more energy. Hence, GDM adds power to the CMB at the scales where oscillation occurs.

Radiation leaking from gravitational potential wells also causes the wells to decay. Hence, increasing (decreasing) decreases (increases) the radiation density of the Universe relative to CDM and therefore decreases (increases) the ISW e ffect due to radiation. This e ffect occurs at scales less than or equal to the size of the Universe, when the energy density of radiation was appreciable. The effects of the GDM model on the CMB can be seen in Figure 1.

Figure 1: The CMB power spectrum predicted by CDM (black dotted line) is compared to that obtained from various GDM scenarios from Kopp et al. Each additional line corresponds to one GDM parameter being varied: (i) the equation of state ( blue ), (ii) the effective sound speed ( green ) or (iii) the viscosity parameter ( red ). The ISW effect is boosted at large scales for the green and red lines. At smaller scales , the effects of changing are visible.

The additional parameters that we have incorporated into the dark matter model make it more flexible. But, flexibility aside, did making dark matter do yoga make it more powerful?

Why does flexibility matter?

The GDM parametrisation is indeed powerful, not just because it allows us to extend the vanilla CDM model, but also because it can be used to model numerous other particle species. If the GDM parameters are assumed to be constant, we can model a cosmological constant, neutrinos and dark radiation, which is a hypothetical form of radiation that mediates interactions between dark matter particles. If they are allowed to vary over time, GDM can model hot and warm dark matter and scalar fields that describe dark energy. Hence, the GDM model allows dark matter to vary its behaviour across di fferent epochs in cosmic history and can better probe the behaviour of particle species such as neutrinos.

Thus, the flexibility of this model does make it more powerful – a very good reason to make dark matter do yoga!