A few recently published articles show that the universe is 13.772 billion (plus or minus 39 million) years old.
That is cool! It is also consistent with some previous measurements of the Universe made in a similar way. Also cool.
What’s not cool is that this doesn’t seem to alleviate the growing discrepancy in measurements in different ways that are becoming a few hundred million years less old. While that may not seem like a big deal, it’s actually a really big deal. Both groups of methods should be of the same age, and they don’t. This means that there is something fundamental about the universe that we are missing.
The new observations were made using the Atacama Cosmology Telescope (or ACT), a six-meter-long dish in Chile that is sensitive to light in the microwave portion of the spectrum, between infrared light and radio waves. When the Universe was very young, it was extremely hot and dense, but about 380,000 years after the Big Bang, it had cooled enough to become transparent. It was then about as hot as the surface of the sun, and the light it emitted would have been more or less in the visible part of the spectrum, the kind of light we see with our eyes.
But the universe has expanded enormously since then. That light has lost a lot of energy to make us fight that expansion, and has shifted red; literally the wavelength has become longer. It is now in the microwave portion of the spectrum. It’s also everywhere, literally in every part of the sky, so we call it the Cosmic Microwave Background, or CMB.
An enormous amount of information is stored in that light, so by scanning the sky with ‘scopes like ACT’ we can measure the conditions in the Universe when it was only 380,000 years old.
ACT covered 15,000 square degrees, more than a third of the entire sky! If they looked at about 5,000 square degrees of that survey, they could determine much of the behavior of the young Universe, including age. Combining that with the results of the Wilkinson Microwave Anisotropy Probe (or WMAP) gave them the age of 13.77 billion years. That also matches the value of the European Planck mission, which also measured microwaves from the early cosmos.
They can also measure the expansion rate of the universe. The expansion was first discovered in the 1920s, and what astronomers discovered is that an object further away from us was withdrawing from us faster. Something twice as far away seemed to move away from us twice as fast. This expansion speed became known as the Hubble constant and is measured in a speed per distance: how fast something is moving versus how far away it is.
The new observations get a value for this constant of 67.6 ± 1.1 kilometers per second / megaparsec (one megaparsec, abbreviated as Mpc, is a unit of distance useful in some aspects of astronomy, equal to 3.26 million light years ; a little further than the distance to the Andromeda Galaxy, if that helps). So, as a result of the cosmic expansion, an object 1 Mpc away from us should disappear at 67.6 km / sec, and a 2 Mpc away at 135.2 km / sec, and so on. It’s a bit more complicated than this, but that’s the gist.
And that’s a problem. There are many ways to measure the Hubble constant – looking at supernovae in distant galaxies, observing gravitational lenses, observing huge gas clouds in distant galaxies, and so on – and many of them get a higher number, about 73 km / sec / Mpc. Those numbers are close to, which is reassuring in some ways, but far enough apart that it’s extremely puzzling. They should agree, and they don’t.
They are also given different ages for the universe. A higher Hubble constant means that the Universe is expanding faster, so it didn’t take that long to reach its current size, making it younger. A lower constant means that the universe is older. So while the rate of expansion may seem esoteric, it is directly linked to the more fundamental concept of how old the universe is, and the two groups of methods are given different numbers.
So what’s right? That’s a tough question to answer, and perhaps the wrong one to ask. A better one is, why do they disagree?
There is an obvious problem, and that is that both methods are correct, but they measure two different parts of the universe. Those looking at the CMB explore the Universe when it was less than a million years old. The others look at the universe when it was already a few billion years old. Perhaps the expansion rate has changed during that time.
In other words, the Hubble constant might not be. A constant, I mean.
There may be problems with the methods themselves, but these have been checked in many ways and with so many different methods in each group that this seems very unlikely at this point.
Apparently the fault is in the universe, and not in ourselves. Or, rather (sorry, Bard, and maybe John), the flaw lies in the way we measure the universe. It does what it does. We just have to find out why.
Many articles have been published about this, and it is no exaggeration to say that it is one of the greatest and most troubling problems in cosmology right now.
A personal thought. My first job after getting promoted was working briefly on a part of COBE, the Cosmic Background Explorer, which looked at the CMB and confirmed that the Big Bang was real. At that time, the measurements were good, but there was room for improvement. Then came WMPA, and Planck, and now ACT, and these measurements have been taken with incredible accuracy. Astronomers call it highly accurate cosmology, kind of an inside joke, since we had barely any idea of these numbers for a long time.
Astronomers are so good at this now that a 10% discrepancy is considered a huge problem, while previously a factor of two was considered ok. It was a real pleasure to see this field improve over time the better we get into it, the better we understand the universe itself as a whole.
Yes, we have some problems. But these are big problems.
Nevertheless, we will hopefully see them resolved soon. Because when we do, it means our understanding has taken another leap forward.