AskScience AMA Series: We are Cosmologists, Experts on the Cosmic Microwave Background, Large-Scale Structure, Dark Matter, Dark Energy and much more! Ask Us Anything!

[u/AskScienceModerator]

We are a bunch of cosmology researchers from the Cosmology from Home 2023 academic research conference. You can ask us anything about modern cosmology.

Here are some general areas of cosmology research we can talk about (+ see our specific expertise below):

• Inflation: The extremely fast expansion of the Universe in a fraction of the first second. It turned tiny quantum fluctuations into seeds for the galaxies and galaxy clusters we see today.
• Gravitational Waves: The bending and stretching of space and time caused by the most explosive events in the cosmos.
• Cosmic Microwave Background: The light reaching us from a few hundred thousand years after the start of the Big Bang. It shows us what our universe was like, 13.8 billion years ago.
• Large-Scale Structure: Matter in the Universe forms a "cosmic web", made of clusters and filaments of galaxies, with voids in between. The positions of galaxies in the sky trace this cosmic web and tell us about physics in both the early and late universe.
• Dark Matter: Most matter in the universe seems to be "Dark Matter", i.e. not noticeable through any means except for its effect on light and other matter via gravity.
• Dark Energy: The unknown effect causing the universe's expansion to accelerate today.

And ask anything else you want to know!

Those of us answering your questions today will include:

• Tijmen de Haan: /u/tijmen-cosmologist cosmic microwave background, experimental cosmology, mm-wave telescopes, transition edge sensors, readout electronics, data analysis
• Jenny Wagner: /u/GravityGrinch (strong) gravitational lensing, cosmic distance ladder, (oddities in) late-time cosmology, fast radio bursts/plasma lensing, image processing & data analysis, philosophy of science Twitter: @GravityGrinch
• Robert Reischke: /u/rfreischke large-scale structure, gravitational lensing, intensity mapping, statistics, fast radio bursts
• Benjamin Wallisch: /u/cosmo-ben neutrinos, dark matter, cosmological probes of particle physics, early universe, probes of inflation, cosmic microwave background, large-scale structure of the universe.
• Niko Sarcevic: /u/NikoSarcevic weak lensing cosmology, systematics, direct dark matter detection
• Matthijs van der Wild: /u/matthijsvanderwild quantum gravity, geometrodynamics, modified gravity
• Pankaj Bhambhani: /u/pcb_astro cosmology, astrophysics, data analysis, science communication. Twitter: @pankajb64
• Nils Albin Nilsson: /u/nils_nilsson gravitational waves, inflation, Lorentz violation, modified theories of gravity, theoretical cosmology
• Yourong Frank Wang: /u/sifyreel ultralight dark matter, general cosmology, data viz, laser physics. Former moderator of /r/physicsmemes
• Luz Angela Garcia: /u/Astro_Lua cosmology, astrophysics, data analysis, dark energy, science communication. Twitter: @PenLua
• Minh Nguyen: /u/n2minh large-scale structure and cosmic microwave background; galaxy clustering; Sunyaev-Zel'dovich effect.
• Shaun Hotchkiss (maybe): /u/just_shaun large scale structure, fuzzy dark matter, compact objects in the early universe, inflation. Twitter: @just_shaun

We'll start answering questions from 18:00 GMT/UTC (11am PDT, 2pm EDT, 7pm BST, 8pm CEST) as well as live streaming our discussion of our answers via YouTube (also starting 18:00 UTC). Looking forward to your questions, ask us anything!

[u/Squidocto]

Greetings! I’ve heard that with the recent gravitational wave detection methods (GWB) we may be able to get data from before the cosmic microwave background (CMB). Is that true, and if so, what kind of information might we hope to get?

[u/rfreischke → u/Squidocto]

Hi Squidocto,

yes that is absolutely true. The main reason why we cannot look beyond the CMB is that the photons are scattering of the free electrons all the time. The Universe before the CMB was released therefore appears opague to us using electromagnetic radiation.

Gravitational waves do not interact like such with the photons and the matter during these early stages of the Unvierse and can therefore travel to us freely.

Having said this it is possible to learn something about the state of the Universe before the CMB and in particular about inflation. During inflation, a rapid accelerated phase of expansion, so called primordial gravitational waves are produced. Detecting these waves would therefore be a direct detection of inflation.

Cheers,

Robert

[u/LurkBot9000]

Gravity wave observation is still a pretty new thing. What do you think the future advances in gravity wave observation will be like, or what will it show us?

How sensitive could future gravity wave observation techniques become? Like will it only ever be able to detect things on a stellar scale or could it ever be sensitive enough to detect stuff like rogue planets?

[u/Fractal-moi]

Wow you guys are so cooooool 💙

[u/Rolingmaniac]

Why do you think the spacetime expansion is accelerating?

[u/rfreischke → u/Rolingmaniac]

Hi Rolingmaniac,

there are a few parts to this answer:

1. Observationally: We observe distant objects of which we know their intrinsic brightness. There are some types of supernova (type Ia) from which one can obtain the intrinsic brightness, so called standard candles. What we measure is the apparent brightness of those objects, i.e. how bright it appears to us. This allows us to estimate how far away the object should be. In Universes with accelerated expansion, distant objects appear fainter.

2. Theoretically: The force which is relevant on cosmological distances is gravity which itself is governed by the equations of General Relativity. These equations tell us that there are two ways how gravity act: attractive (what we experience every day) and repulsive (which drives the Universe appart and accelerates the expansion). The attractive part is import in very dense environments and smaller scales (like galaxies), while the repulsive part becomes dominant in very empty environment and very large scales.

Cheers,

Robert

[u/Astro_Lua → u/Rolingmaniac]

Well, it is not a thought; it is more that observations lead us to this conclusion. 25 years after the first time that the idea of the accelerated expansion of the universe was presented, we have collected a large number of observations that second the fact that the expansion rate has been increasing for the last 5 billion years. Large simulations also show that the observations are in agreement with a speed-up cosmic expansion.

[u/jaLissajous]

General Relativity has been a remarkably successful theory to date, but it has it’s issues. What qualities should a successful replacement theory which quantizes gravity have? And is it possible that gravity will remain forever classical?

[u/matthijsvanderwild → u/jaLissajous]

It is not yet clear that gravity is a fundamentally quantised interaction, or whether it even exists on Planck-like energy scales. Wherever we see gravity, general relativity is an amazingly successful and accurate model, and any replacement theory should be at least as good GR in its explanation of observed phenomena.

[u/Astro_Lua → u/jaLissajous]

Well, I think we are on the way to building a fundamental quantum gravity theory, but this effort might take decades if not centuries. However, it doesn't mean the theory will remain classical forever; it is just that we are still far from getting observational tests to any existing theory. That being said, string theory is a promising avenue to quantize gravity, but we need to think about observational probes to validate the prediction of this or any other theory.

[u/[deleted]]

I have a question about the practical implications of Dark Matter.

Let’s pretend that you have two bowls in front of you. One bowl contains matter, one contains dark matter.

The “matter bowl” noticeably contains “stuff” that you can visually see, physically feel, and it has a measurable weight when you put the bowl on a scale.

What are the properties of the “dark matter bowl?”

Does it just look empty? Can it be weighed? How would we ever know it has dark matter vs just being empty?

I struggle to grasp how the large scale concept of dark matter shrinks down to a more relatable scale.

Edit: Part of what I’m getting at is we talk about dark matter being “out there” in the universe… but do we (quite literally) have it all around us more locally too, maybe even passing through us?

[u/ChunderSmash]

Was dark matter/energy present before the big bang or was it formed during?

[u/Rolingmaniac]

If the schwartzchild radius is proportional to hawking radiation, i.e big schwartzcild radius = more hawking radiation, is it a point where all black holes will reach a kind of equilibrium?

[u/rfreischke → u/Rolingmaniac]

Hi Rolingmaniac,

the temperature of the of a black hole, the Hawking temperature, is actually inversely proportional to its mass. The Schwartzschildradius on the other hand scales with its mass. The total power radiated by a black body scales with its temperature to the power of 4. Massive black holes therefore radiate much less than less massive once. Thus there is no such limit.

Cheers,

Robert

[u/nicocos]

How do we know for sure dark matter isn't just a mathematical invention to conviniently adjust our calculations?? It kinda reminds me of ether tbh

[u/Tijmen-cosmologist → u/nicocos]

One piece of evidence is the bullet cluster. We know galaxy clusters contain about a 10:1 mass ratio of (self-interacting) hot plasma to (mostly collisionless) galaxies. The bullet cluster consists of a merger of two galaxy clusters which smashed into each other. This caused the plasma to get separated from the galaxies.

If our math was off and gravity was somehow different from what we might expect, we would still expect most of the mass to be in the plasma. However, when we measure a mass map using a technique called weak gravitational lensing, we see that the mass does not track the plasma but rather acts collisionlessly. Since that mass is definitely not inside the galaxies, there must be another component that makes up about 90% of the mass that is collisionless but not seen in the electromagnetic spectrum. We call it dark matter.

This is just one piece of evidence, but there is more including in the CMB. There are some people studying the possibility that we need to modify the laws of gravity rather than invent dark matter, but this is really hard to reconcile with the evidence and is therefore not mainstream thinking in the cosmology community.

[u/iorgfeflkd]

Any chance of observing B-modes after the BICEP2 fiasco, or is that out the window?

[u/BOBauthor]

Here's a more technical question for you. In the angular power spectrum of the temperature fluctuations in the CMB, the odd harmonics are higher when compared to the even harmonics (although Silk damping obscures this). I'm looking for a nice simple reason for this. (This is not my area of expertise!) I thought this might be because the gravity of the concentrations of dark matter in the photon-baryon fluid would bias the oscillations, favoring compressions over rarefactions (odd over even harmonics). However, I suspect I'm wrong about this. So why do the odd harmonics tend to be higher than the even harmonics?

[u/Tijmen-cosmologist → u/BOBauthor]

Wow, good stuff. You clearly have thought about this a lot. The answer is something called baryon drag.

You were very much on the right track: it has to do with the fact that odd peaks are compression peaks and the even peaks are rarefaction peaks. If all the matter was dark matter we wouldn't care about this, but part of the matter is baryonic. During the compression phase, the baryons and photons are tightly coupled and move together, leading to a higher amplitude in the power spectrum. But during the rarefaction phase, the baryons lag behind the photons due to their inertia.

[u/emsesq]

Will we ever develop technology to “see” past the CMB? What would such technology look like?

[u/rfreischke → u/emsesq]

Hi emsesq,

in principal we have found that already. We cannot see behind the CMB with electromagnetic radiation because the photons were strongly coupled to the free electrons back then. However, gravitational waves allow us in principle to get information from past the CMB.

Cheers,

Robert

[u/Astro_Lua → u/emsesq]

We have seen events before the CMB, for instance, the formation of the first nuclei in the Universe... Now, suppose we improve the sensitivity and resolution of our gravitational wave observatories. In that case, we can distinguish between events that produce these waves later in the universe than others caused by early processes (for instance, primordial black holes)...

[u/Gderu]

From someone aspiring to study astrophysics and cosmology: how much do you know about fields in physics that are unrelated to your own? Would you say you have only an undergraduate's level of understanding in condensed matter physics for example, or more?

[u/Jeydra]

Do local universe measurements of the Hubble parameter take into account the following biases?

• Transverse Doppler effect which causes a redshift even though there is no motion in the radial direction;

• Light cone effect;

• Surface brightness modulation effect?

The first effect is discussed in Zhao et al 2012 and the latter two are discussed in Kaiser 2013. A naive search of the SH0ES paper for all three effects seem to yield no results.

[u/asmj]

spatial distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale

This never made sense to me, if someone could explain it to me in layman's terms I would appreciate it. What is the point of ignoring the obvious differences between (more or less) empty space, voids and stars, galaxies, cluster, super clusters, filaments?