午夜福利1000集合

IT IS nearly 100 years since we confirmed that the universe 鈥� space-time 鈥� is expanding. But we are still struggling with a basic fact: what is the rate of the expansion? Depending on how we measure a crucial number that sets this value, we seem to get different answers. The fallout of this question could drastically change our understanding of the cosmos.

In 1929, astronomer Edwin Hubble used observations of galaxies to show that there was a correlation between their velocity and their distance from us. The further away they were, the faster they seemed to be receding from our galaxy. General relativity, which at that point had only been around for a decade and a half, had a clear theoretical explanation for this finding: space-time isn鈥檛 static. It expands, carrying galaxies along like a raft on a river.

Hubble was able to make this radical discovery because of something we now call Leavitt鈥檚 law. Discovered by Henrietta Leavitt in 1908, the law concerns young stars called Cepheid variables. These stars are called variable because their brightness and size vary.

While working as a computer at Harvard College Observatory, Leavitt noticed that Cepheids had a pattern: the power of their light emission 鈥� absolute magnitude, to astronomers 鈥� correlates with the frequency of the pulsations. In other words, by observing these, one could calculate their absolute magnitude, which we could then use to calculate how far we are from an object.

Leavitt鈥檚 law created a rung on what we call the cosmological distance ladder, which is a collection of different ways that we measure distances to objects in the sky. Cepheids proved to be a powerful tool because they are found in other galaxies. Hubble took advantage of that, leading to his finding that galaxies move away from us at a velocity directly proportional to their distance from us. The proportionality constant of this relationship is known as the Hubble constant.

Ninety years after Leavitt discovered her law, astronomers were making distance measurements using exploding stars known as type Ia supernovae. They found something unexpected: not only is space-time expanding, but that expansion is accelerating, like a river current picking up speed. The discovery of cosmic acceleration, as this phenomenon is known, netted three of the astronomers involved a Nobel prize.

鈥淭he discovery of cosmic acceleration netted three of the astronomers involved a Nobel prize鈥�

For a time, discussions of expansion focused primarily on the problem of explaining cosmic acceleration. But now astronomers are spending a lot of time arguing over the exact value of Hubble鈥檚 constant.

Adam Riess, one of the cosmic acceleration Nobel laureates, has been leading a team that used type Ia supernova to find a value for the Hubble constant that is 12 per cent larger than one produced by a different method involving the cosmic microwave background (CMB) radiation, the leftover radiation from the big bang. This difference is much larger than the margin of error involved, which means that the two measurements appear to be in disagreement, assuming they are taking all of the right physics into account correctly.

As wonderful as Cepheids are, because they are young stars, they tend to be surrounded by dust. This is a problem because it can obscure measurements, making them inaccurate. It means there may be errors in Cepheid-based calibrations of type Ia supernova distances, leading to an error in the distances calculated from observations of them.

In fact, the Panchromatic Hubble Andromeda Treasury collaboration (PHAT) has since found that Riess and his team may have the wrong calibrations for their Cepheids and this may be partly due to their use of ground-based telescopes as opposed to something in space like PHAT uses. The callibrations also suffer from crowding 鈥� the possibility that what looks like one star is actually several 鈥� and this effect can get worse with distance.

In July 2019, astronomer Wendy Freedman said these inconsistencies are 鈥渨hat keeps [her] up at night鈥� a team that is using another type of star to measure the Hubble constant: those in the so-called tip of the red giant branch, known as TRGBs. These stars are maybe half the mass of the sun, and are at a later stage of their lives where helium burning has begun in their cores, after all of their hydrogen has burned up. Earlier this year, Freedman鈥檚 team published a paper saying that calibrating type Ia supernovae with TRGBs leads to a Hubble value that is somewhere between that suggested by CMB measurements and what Riess鈥檚 team has found.

Why are these values so different? We just don鈥檛 know 鈥� and not knowing is part of what makes science so fun.