[Harri (OP)]

I was just reading about the Atacama Cosmology Telescope in Chile. Research is as active as ever in trying to determine the age of the universe and the rate at which the universe is expanding. Does this research measure the rate at which matter is travelling through the universe or measure the actual expansion of the fabric of the universe?

[Halc]

I was just reading about the Atacama Cosmology Telescope in Chile. Research is as active as ever in trying to determine the age of the universe and the rate at which the universe is expanding. Does this research measure the rate at which matter is travelling through the universe or measure the actual expansion of the fabric of the universe?

The research is intended to determine the actual expansion rates of the 'fabric' of the universe, but since that cannot be measured directly, they instead measure the rate of increase of proper distance between us and distant galaxies over time.  These distant galaxies are generally not travelling very fast through the universe (usually less than 0.02c) relative to said fabric, and thus make a good way to indirectly measure the expansion rate.

The rate has not been constant over time, so for instance some distant galaxy might be receding today at twice light speed, but 6 billion years ago it might have been receding at 1.9c and 13 billion years ago it might have been receding at 3c.  Expansion had been slowing considerably since the early times, and measuring those rates from all the points along that curve is critical to estimating the age of the universe.  Problem is, for any one galaxy, one can only see it at one moment in time, depending on how far away it was then.

[evan_au]

This telescope is trying to measure the CMB with very precise angular resolution across the sky.
- Some "Hotspots" in the CMB are due to galaxy clusters, whose high-energy electrons interact with the CMB to increase its apparent temperature (by millionths of a degree Kelvin)
- By identifying these distant galaxy clusters, they can take a measure of the expansion of the universe
- One unique thing about using the Sunyaev-Zeldovich effect is that it does not suffer from the inverse square law, like a normal telescope
- It is able to detect galaxy clusters with a redshift of 3 almost as easy as galaxy clusters with redshift of 0.2
- My simplistic understanding is that more distant objects made up a larger part of the universe (at the time the light was emitted), and so leave an imprint on the CMB that is as large as the imprint of nearer objects(?)
- A better explanation would be appreciated!

See: https://en.wikipedia.org/wiki/Atacama_Cosmology_Telescope
https://en.wikipedia.org/wiki/Sunyaev%E2%80%93Zeldovich_effect
 

[Bored chemist]

If I was watching a video of a river, I couldn't actually see how fast the water was moving.
But, if I watched the fish + bits of weed +twigs and then averaged the speeds that they were moving, I'd get a fair idea of the water speed.

[Harri (OP)]

Is there anything at all to suggest that the increasing rate of expansion could be due to cooling? Expansion I know is usually attributed to heating but is there anything that expands with cooling?

[Halc]

Is there anything at all to suggest that the increasing rate of expansion could be due to cooling?

Other way around: Cooling is due to expansion, regardless of if the expansion rate is increasing or decreasing.

Expansion I know is usually attributed to heating but is there anything that expands with cooling?

An idea gas will cool if expanded, and heat if compressed, and yes, if heat is injected from outside the system, a gas will expand, but there is no outside of the universe from where new energy can be injected.
The heat shields for spacecraft coming out of orbit guard against the head of air being compressed ahead of the object coming in, and not so much against friction which is a far smaller heating effect.
Treating the universe as an expanding idea gas is probably a poor analogy. The CMB appears cooler because of redshift. That light is still as energetic as the day it was emitted in the frame in which the emitting material is stationary.  In this sense, the universe isn't cooling, it just appears that way due to relative motion between observer and the observed.

As for the current acceleration of expansion:
There are multiple factors pushing on the universe.  One is gravity.  Early on, everything is closer together and there is greater mutual attraction between all mass.  This tends to slow down expansion, but it is a function of proximity, so that effect decreases as the universe becomes less dense.  The other major contributor is dark energy which is a function of the volume of the universe.  It becomes greater as the relative volume increases.  About 6 billion years ago, those two exactly cancelled out, and expansion stopped slowing.  But the former (gravity drag) is still decreasing, and volume is still increasing, so at that point the universe entered an epoch dominated by dark energy, and expansion began to accelerate.  It is accelerating now, and always at a greater and greater rate.

[evan_au]

Treating the universe as an expanding ideal gas is probably a poor analogy. The CMB...

As a gas expands, the relative speed (and kinetic energy) of the gas molecules reduces, and temperature is directly proportional to energy.
- The CMB is electromagnetic radiation, and its relative speed is always c. However, red shift reduces the frequency, and frequency is directly proportional to energy, which is directly proportional to temperature.

One area where an expanding gas model may apply is to Dark Matter (assuming it is some form of weakly interacting subatomic particle):
- In the high density of the Big Bang, Dark Matter would have had a very high temperature
- As the universe expanded, the relative velocity of the Dark Matter has reduced
- To the extent that Dark Matter can now be captured in orbit around our galaxy, forming a Dark Matter halo (if it still had a high temperature, it would exceed the galaxy's escape velocity, and there would not be a Dark Matter halo to explain anomalous rotation curves)
See: https://en.wikipedia.org/wiki/Cold_dark_matter