“Hell must be isothermal; for otherwise the resident engineers and physical chemists (of which there must be some) could set up a heat engine to run a refrigerator to cool off a portion of their surroundings to any desired temperature.” -Henry Albert Ben
One of the most amazing ideas to come out of our observations of the Universe over the last century is that our vast, star-filled, mostly-empty Universe hasn’t always been like this.
Today, the Universe is very cold, expanding, and the average distance between galaxies is increasing as time goes on. The farther away a galaxy is, the faster it appears to be moving away from us and the farther towards the red end of the spectrum its light appears to be shifted.
The Big Bang, of course, is the theoretical framework that makes sense of all these observations, and leads us to our present picture that the Universe was much hotter and denser in the past, and has been cooling, expanding and diluting ever since, something that continues at this very moment.
The clutch confirmation of this theory came in the mid-1960s, when the ultimate prediction of the Big Bang — that a background of blackbody radiation just a few degrees above absolute zero should permeate the entire Universe — was detected equally, omnidirectionally throughout the Universe by Arno Penzias and Bob Wilson.
This radiation is everywhere, in all directions, and is as close to uniform as you’re likely to find in nature. Even though the photo you’ve most likely seen of it — the Cosmic Microwave Background (CMB) radiation — looks anything but uniform.
That’s because what you’re looking at here are the fluctuations in the CMB, or the departures from perfect uniformity. The reality of the situation is that the average temperature of the CMB is 2.725 K, and the largest fluctuations you can see are maybe just 100 microKelvin, or on a scale of 0.0001 K!
For comparison, that would be like showing a map of the elevation of the Earth, if the highest mountain on Earth, instead of being Mount Everest, was the hill that comes as your default MS Windows background.
In other words, it’s really uniform! And like I said, it isn’t just the fact that it’s a uniform temperature in all directions, it’s that it follows a very particular distribution: the blackbody spectrum. This was confirmed more than 20 years ago by the COBE satellite, in one of the most strikingly accurate matches between theory and observations of all-time.
What you might notice is that even though this radiation peaks at around millimeter-scale wavelengths (placing it firmly in the microwave region), it extends to both shorter wavelengths (into the infrared) and far out to longer and longer wavelengths, going far into the radio portion of the electromagnetic spectrum.
The wavelengths get so long (and the frequencies low enough) that if you can find an old television set with rabbit-ear-antennae and set it to channel 3, about 1% of the “static” that you see comes from the cosmic microwave background!
This static would be there even if you left Earth, left the Sun and Solar System behind, and even wiped out all the galaxies from the sky. That’s the static that’s left over from the Big Bang.
And as the Universe continues to age and expand, it also continues to cool. Which means, if we look back into the past, that leftover radiation should have been hotter back then. Big Bang cosmology even predicts exactly how much hotter it should have been: if you can measure the redshift (z) of a distant galaxy, the temperature of the CMB back then should have been exactly what it is now, multiplied by 1 + z. And the data seems to match, as far as we can tell!
But even at their very best, the techniques used to date still have large error bars on them, particularly at high redshift. But researchers from CSIRO’s Australia Telescope Compact Array have just made the best-ever determination of whether this relation holds true at high redshift or not.
When you head out to the outskirts of a galaxy — away from the stars or dusty areas within — the only thing keeping the temperature of the gas there from dropping asymptotically toward absolute zero is the CMB itself. By looking at a particular distant galaxy located 7.2 billion light years away (at a redshift of z=0.89), scientists would expect to find gas at a temperature of 5.14 K, nearly double the 2.725 K we observe today.
Of course, that’s far too low a temperature for us to detect it with an array of radio telescopes at this phenomenal distance. But if there were a strong energy source behind this galaxy, relative to our line-of-sight, we could infer the temperature based on the effects of the intervening gas from the galaxy itself.
This one particular galaxy was a pristine candidate for these observations, and the researchers (S. Muller et al., here) were able to determine that the temperature of this gas — in equilibrium with the CMB at that redshift — is 5.08 ± 0.10 K, in outstanding agreement with the Big Bang’s prediction of 5.14 K and the best-ever measurement of temperature at such a high redshift! (New point in red, below.)
And there you have it. The Universe has been expanding and cooling for over 13 billion years. It’s cooler right now than it’s ever been before, and is the warmest it will ever be as we continue to move forward in time. This is a brand new way of testing the Big Bang to this level of accuracy, and once again, the observations show that the theory passes the test with flying colors.
Well done to the entire team that made this happen, and now you know just one more way that we can confirm the Big Bang: by looking back to a time when the Universe was twice as hot!