Saturday, May 21, 2022

Hubble is closer to solving one of astronomy's great mysteries

The Hubble Space Telescope has completed a nearly 30-year cycle of observing cosmic objects known as “standard candles” — Cepheid stars and Type Ia supernovae . With this, astronomers now have an unprecedented set of data to calculate the speed of expansion of the universe

The Hubble constant problem

In 1924, the American astronomer Edwin Hubble discovered that there were many other galaxies besides ours and found that they were in constant distance from each other. The further away the galaxies were, the faster they were moving away. Hence came Hubble's law, in which we can calculate the speed of this expansion.

Hubble created a unit that describes how fast the universe is expanding, which is in (km/s)/Mpc. At the time, Hubble measured the value at 501 km/s for 1 Mpc (Megaparsec, equivalent to 3.26 million light years), that is, galaxies 1 Mpc away would have a speed of 501 km/s. This result, however, was surrounded by uncertainties.

When the Hubble telescope was planned, this was still a problem. So astronomers planned to use the new instrument to determine once and for all the expansion rate of the universe. This could be done if the telescope could collect accurate data from Cepheid stars and Type Ia supernovae.

 That's exactly what the Hubble telescope did during its first marathons of observations — thanks to the unspeakable efforts of decades of research carried out by different teams. Despite this, the Hubble constant problem has not yet been solved.

The Hubble Telescope looks for a solution

Shortly after the launch of the Hubble Space Telescope in 1990 came the first batch of observations of Cepheid stars to calculate the Hubble constant. For this, two teams were needed: the HST Key Project and the team led by Allan Sandage. Cepheids are stars that increase in size periodically, which allows you to calculate their distances with high accuracy.

Knowing the distances to the Cepheids and measuring them periodically, astronomers should be able to tell the rate at which the universe is expanding, as they are moving away due to this phenomenon. But in practice, it is not so simple: other equally accurate methods of calculating the expansion of the universe yield different results.

In the early 2000s, teams studying Cepheids through the Hubble telescope declared it "mission accomplished". They got a value of 72 (km/s)Mpc and a margin of error of only 10% — for comparison, the estimates before the telescope had a margin of error of 50%.

Despite giving some confidence to scientists, the 10% margin is still not as satisfactory as we would like. So in 2005, and then in 2009, new cameras were added to the space telescope, which ushered in the "Generation 2" search for the Hubble constant. The goal: get results with 99% certainty.

Project SH0ES (acronym for Supernova, H0, for the Equation of State of Dark Energy) was one of the new teams formed to calculate the number by studying the cosmic microwave background radiation (the “light” left over from the Big Bang, like a fossil from the beginning of the universe).

Several teams participated in the efforts and the data pointed to a value of 73 (km/s)Mpc. Other approaches were used to measure the Hubble constant and arrived at approximate results. Even so, the debate is far from over, and it causes a headache for physicists.

Unfortunately (or not), measurements from the European Space Agency's Planck mission (which also observed microwave background radiation) predict a lower value for the Hubble constant: 67.5 (km/s)Mpc. Some say that this divergence would be a “non-problem” , but the concern remains.

And now?

Nearly 30 years after the first Hubble constant surveys with the eponymous telescope, the SH0ES team measured 42 new standard candles — all of them Type Ia supernovae, that is, stars exploding at a rate of about once a year.

"We have a complete sample of all Hubble-accessible supernovae seen in the last 40 years," said Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University. He is the leader of the new study, accepted for publication in The Astrophysical Journal .

Astronomers attribute the expansion of the universe to something known as dark energy.

"The Hubble constant is a very special number," said Dr. Licia Verde, a cosmologist at ICREA and the ICC-University of Barcelona. our understanding of the universe. This required a phenomenal amount of detailed work".

With the Hubble sample count, “there's only a one-in-a-million chance astronomers are unlucky enough to be wrong,” Riess said of the estimates the standard candlestick catalog can provide once complete. “I don't actually care about the expansion value specifically, but I like to use it to learn about the universe,” he added.

Perhaps Riess and his colleagues are well rewarded for their 30 years of work on expanding the universe with the Hubble telescope. It is that, depending on the results to be obtained with a new set of supernovae, scientists may arrive at the discovery of a completely new physics. This is something worth investing a career in. 

Collection of 36 Hubble Telescope images; each of these galaxies hosts Cepheid variables and supernovae (Image: Reproduction/NASA/ESA/Adam G. Riess (STScI, JHU)

Source: NASA

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