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A pair of recently published papers show that the Universe is 13.772 billion (plus or minus 39 million) years old.
That’s great! It is also in agreement with some previous measurements of the Universe made in a similar fashion. Also cool.
what do not cool is that it doesn’t seem to close a growing gap in actions taken in different ways that save a few hundred million years less. While this might not seem like a big deal, it’s actually a really big deal. Both groups of methods should be the same age, and they are not. This means that there is something fundamental in the Universe that we are missing.
The new observations were made using the Atacama Cosmology Telescope (or ACT), a six-meter 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 after about 380,000 years after the Big Bang it has cooled enough to become transparent. It was about as hot as the surface of the Sun back then, and the light it was emitting 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 grown a lot since then. This light has lost a lot of energy to get us to fight against this expansion, and has changed speed; literally, the wavelength has lengthened. It is now in the microwave part of the spectrum. It’s also everywhere, literally in every part of the sky, so we call it the microwave cosmic background, or CMB.
A huge amount of information is stored in this light, so by scanning the sky with “extents like ACT, we can measure conditions in the Universe when it was only 380,000 years old.”
ACT covered 15,000 square degrees, or over a third of the entire sky! By looking at about 5,000 square degrees from this survey, they were able to determine much of the behavior of the young Universe, including its age. Combining this with the results from the Wilkinson Microwave Anisotropy Probe (or WMAP), they reached the age of 13.77 billion years. This also dovetails with the value of the European Planck mission, which also measured microwaves from the early cosmos.
They can also measure the rate of expansion of the Universe. The expansion was first discovered in the 1920s, and astronomers discovered that an object farther from us was moving away from us faster. Something twice as far seemed to be moving away from us twice as fast. This rate of expansion has come to be known as the Hubble constant, and it’s measured in speed per distance: how fast something is moving relative to its distance.
The new observations obtain a value for this constant of 67.6 ± 1.1 kilometers per second / megaparsec (one megaparsec, abbreviated Mpc, is a unit of distance practical in some aspects of astronomy, equal to 3.26 million light years away; a little further than the distance to the Andromeda Galaxy, if that helps). So, due to cosmic expansion, an object 1 Mpc far away should move away from us at 67.6 km / s, and a 2 Mpc object twice as far away at 135.2 km / s, etc. It’s a little more complicated than that, but that’s the gist.
And that’s a problem. There are many ways to measure the Hubble constant – by looking at supernovae in distant galaxies, by observing gravitational lenses, by observing huge clouds of gas in distant galaxies, etc. – and many of them get a larger number, around 73 km / sec / Mpc. These numbers are To close, which is reassuring in some ways, but remote enough to be extremely confusing. They should agree and no.
They also have different ages for the Universe. A higher Hubble constant means the Universe is growing faster, so it doesn’t need that long to reach its current size, making it younger. A lower constant means the Universe is older. So, although the rate of expansion may seem esoteric, it is directly related to the more fundamental concept of the age of the Universe, and the two groups of methods get different numbers.
So what is fair? This is a difficult question to answer, and perhaps the wrong one to ask. A better is, why don’t they agree?
There is an obvious problem, and that is that these two methods are correct, but they measure two different parts of the Universe. Those who watch the CMB are looking at the Universe when it was less than a million years old. The others look at the Universe when there were already some billion years. Maybe the rate of expansion changed during this time.
In other words, maybe the Hubble constant is not. A constant, I mean.
There might be issues in the methods themselves, but these have been checked in so many ways and by so many different methods in each group that it seems highly unlikely at this point.
The fault is apparently in the Universe, and not in ourselves. Or, better said (sorry, Bard, and maybe John), the fault lies in the way we measure the Universe. He does what he does. We just need to understand why.
Many articles have been published on this subject, and it is no exaggeration to say that it is one of the most important and thorniest issues in cosmology today.
A personal thought. My first job after getting my PhD was briefly to work on part of COBE, the Cosmic Background Explorer, which examined the CMB and confirmed that the Big Bang was real. At that time, the measurements were good, but there was room for improvement. Then WMPA came along, and Planck, and now ACT, and these measurements are done with incredible precision. Astronomers call it high-precision cosmology, kind of inside joke since for a long time we had almost no idea what those numbers were.
Astronomers are so good at it now that a 10% deviation is considered a huge problem, when back then a factor of two was considered OK. Watching this area improve over time has been a real joy because the better we do it, the better we understand the universe itself as a whole.
Yeah, we have problems. But these are big problems to have.
Nevertheless, I hope we will see them resolved soon. Because when we do, it means that our understanding will have taken another big step forward.
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