A couple of recently published papers show that the Universe is 13.772 billion (roughly 39 million) years old.
It is great! It also coincides with some previous measurements of the Universe made in a similar way. Also cool.
What is? no Most interestingly, this does not seem to alleviate a growing discrepancy in measurements made in different ways, reaching an age of hundreds of millions of years less. While this may 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 lack.
The new observations were made with the Atacama Cosmology Telescope (or ACT), a six-meter dish in Chile that is sensitive to light in the microwave part of the spectrum, between infrared light and radio waves. When the Universe was very young it was very hot and dense, but after about 380,000 years after the Big Bang it cooled down enough to become transparent. It was as hot as the surface of the Sun at that time 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 a lot since then. This light has lost a lot of energy getting us to fight this expansion and has shifted to red; literally, the wavelength has been lengthened. It is now in the microwave spectrum. It is also everywhere, literally everywhere in the sky, so we call it Cosmic Microwave Background or CMB.
A huge amount of information is stored in this light, so by scanning the sky with areas like ACT, we can measure the conditions of the Universe when it was only 380,000 years old.
ACT covered 15,000 square degrees, more than a third of the entire sky. Observing about 5,000 square degrees of this survey, they were able to determine a large amount of the behavior of the young Universe, including its age. Combining this with the results of the Wilkinson Microwave Anisotropy Probe (or WMAP), they obtained the age of 13,777 million years. This also agrees with the value of the European Planck mission, which also measured microwaves from the first cosmos.
They can also measure the rate of expansion of the Universe. Expansion was first discovered in the 1920s and what astronomers found is that an object farther away from us was moving away from us faster. Something twice as far seemed to get away from us twice as fast. This speed of expansion became known as the Hubble constant, and is measured at a speed per distance: the speed at which something moves and the distance it has.
The new observations obtain a value for this constant of 67.6 ± 1.1 kilometers per second / megaparsec (a megaparsec, abbreviated as Mpc, is a unit of convenient distance 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). Therefore, due to cosmic expansion, an object at 1 Mpc away should withdraw us at 67.6 km / sec, and at 2 Mpc twice that at 135.2 km / sec, and so on. It’s a little trickier than that, but that’s the bottom line.
And that is a problem. There are many ways to measure the Hubble constant: looking at supernovae in distant galaxies, observing gravitational lenses, observing huge clouds of gas in distant galaxies, and so on. seg / Mpc. These numbers are Close, which is reassuring in some ways, but far enough away to make it extremely baffling. They should agree, and no.
They also have 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 the current size, making it younger. A lower constant means that the Universe is older. Thus, although the rate of expansion may seem esoteric, it is directly linked to the most fundamental concept of how old the Universe is and the two groups of methods obtain different numbers.
What is the case, then? This is a difficult question to answer and perhaps the wrong one to ask. One of the best is, why do they disagree?
There is an obvious problem, and that is that both methods are correct, however they measure two different parts of the Universe. Those looking at the CMB are examining the Universe when it was less than a million years old. The others look at the Universe when there were already a few billion years. Perhaps the rate of expansion changed during this time.
In other words, perhaps the Hubble constant is not. A constant, I mean.
There could be problems in the methods themselves, but these have been proven in many ways and by so many different methods in each group that it seems very unlikely at this time.
Apparently, the blame lies in the Universe and not in ourselves. Or rather (sorry, Bard and maybe John), the blame lies in the way we measure the Universe. He is doing what he does. We just have to figure out why.
Many articles have been published on this, and it is no exaggeration to say that it is one of the biggest and thorniest problems in cosmology at the moment.
A personal thought. My first job after getting my PhD was briefly working on a part of COBE, the Cosmic Background Explorer, which examined the CMB and confirmed that the Big Bang was real. At that time the measures were good, but there was room for improvement. Then came WMPA, Planck and now ACT, and these measurements are done with incredible accuracy. Astronomers call it high-precision cosmology, a kind of inner joke because, for a long time, we barely had any idea of these numbers.
Astronomers are so good at this now that a 10% discrepancy is considered a huge problem, when in the past a factor of two was considered correct. Seeing this field improve over time has been a real joy, because 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.
Still, we hope to see them resolved soon. Because when we do, it means that our understanding will have taken another big leap.