Hubble and Gaia associate to feed the cosmic enigma

Using the power and synergy of two space telescopes, astronomers have made the most accurate measurement to date of the rate of expansion of the universe.

The results further fuel the discordance between Near-Universe Expansion Rate measurements and those of the far-off and primitive universe – even before stars and galaxies exist.

This "tension" implies that there could be a new physics underlying the foundations of the universe. Possibilities include the force of dark matter interaction, black energy being even more exotic than previously thought or a new unknown particle in the tapestry of space [19659005] Combination observations of NASA's Hubble Space Telescope and the ESA Observatory, astronomers have refined the previous value for the Hubble constant, the speed at which the universe is expanding since the big bang 13.8 billion years ago.

But the measurements becoming more precise, the determination of the Hubble constant increasingly contradicts the measurements of another space observatory, the Planck mission of the ESA, which predicts a predicted value different for the Hubble constant.

Planck mapped the primitive universe as it appeared only 360,000 years after the big bang. . The whole sky is stamped with the signature of the Big Bang encoded in microwaves. Planck measured the sizes of the ripples in this cosmic microwave background (CMB) that were produced by slight irregularities in the big bang fireball. The small details of these ripples encode the amount of dark matter and normal matter, the trajectory of the universe at that time and other cosmological parameters.

These measurements, still under evaluation, allow scientists to predict how the primitive universe probably evolved towards the rate of expansion that we can measure today. However, these predictions do not seem to correspond to the new measures of our near contemporary universe.

"With the addition of these new data from the Gaia and Hubble Space Telescope, we now have a serious voltage with the Cosmic Microwave Background data," said Planck team member and senior badyst George Efstathiou of the Kavli Institute of Cosmology in Cambridge, England, who was not involved in the new work.

"Tension seems to have become a full-fledged incompatibility between our views," said team leader and Nobel laureate Adam Riess of the Space Telescope Science Institute and Johns Hopkins University Baltimore, Maryland. "At this point, it's clear that it's not just a gross mistake to a degree, it's as if you're predicting how much a child would grow up from a curve of growth to then discover that the adult far exceeded the prediction. "

In 2005, Riess and members of the team SHOES (Supernova H0 for the State Equation) undertook to measure the rate of expansion of the universe with unprecedented accuracy. In subsequent years, refining their techniques, this team has reduced the uncertainty of measuring rates to unprecedented levels. Now, with the power of Hubble and Gaia combined, they have reduced this uncertainty to only 2.2%.

Because the Hubble constant is needed to estimate the age of the universe, the answer sought is one of the most important numbers in cosmology. It takes its name from the astronomer Edwin Hubble who, nearly a century ago, discovered that the universe was spreading uniformly in all directions – a discovery that gave birth to the modern cosmology.

Galaxies appear to retreat from Earth in proportion to their distances. the further away they are, the more quickly they seem to move away. This is a consequence of the expansion of space, and not a value of true space velocity. By measuring the value of the Hubble constant over time, astronomers can construct an image of our cosmic evolution, infer the composition of the universe, and uncover clues about its ultimate destiny.

The two main methods of measuring this number inconsistent results. One method is direct, building a cosmic "distance scale" from the measurements of the stars in our local universe. The other method uses the CMB to measure the trajectory of the universe shortly after the big bang and then uses physics to describe the universe and extrapolate at the current rate of expansion. Together, the measurements should provide an end-to-end test of our basic understanding of the so-called "standard model" of the universe. Using Hubble and Gaia's recently released data, the Riess team measured the current rate of expansion at 73.5 kilometers (45.6 miles) per second per megaparsec. This means that for every 3.3 million more distant light years, a galaxy belongs to us, it seems to move faster by 73.5 kilometers per second. However, Planck's results predict that the universe is expected to be expanding today at just 67.0 kilometers (41.6 miles) per second per megaparsec. As teams' measurements have become increasingly precise, the gap between them has steadily widened and now represents about four times the size of their uncertainty.

Over the years, the Riess team has refined Hubble 's constant value by rationalizing and reinforcing the cosmic distance scale, used to measure precise distances to nearby and distant galaxies. They compared these distances to the expansion of space, as measured by the light stretching of neighboring galaxies. Using apparent velocity at each distance, they then calculated the Hubble constant.

To measure distances between nearby galaxies, his team used a special type of star as a cosmic landmark or milestone marker. These pulsating stars, called Cephied variables, illuminate and darken at frequencies corresponding to their intrinsic luminosity. By comparing their intrinsic luminosity with their apparent luminosity as seen from Earth, scientists can calculate their distances.

Gaia refined this measure by geometrically measuring the distance to 50 Cepheid variables of the Milky Way. These measurements were combined with precise measurements of their Hubble luminosity. This allowed astronomers to more precisely calibrate Cepheids, then use those seen outside the Milky Way as milestone markers

"When using Cepheids, you need distance and of brightness, "explains Riess. Hubble provided information on brightness, and Gaia provided the parallax information needed to accurately determine distances. The parallax is the apparent change in the position of an object due to a change in the observer's point of view. The ancient Greeks first used this technique to measure the distance between the Earth and the Moon.

"Hubble is really a general purpose observatory, but Gaia is the new benchmark in calibration distance. "That's what it was designed for," added Stefano Casertano of the Space Telescope Science Institute and member of the SHOES team. "Gaia brings a new ability to recalibrate all past distance measurements, and this seems to confirm our previous work: we get the same answer for the Hubble constant if we replace all previous calibrations from the distance scale by the Gaia parallaxes between two very powerful and precise observatories. "

The goal of the Riess team is to work with Gaia to cross the threshold of refining the Hubble constant to a value of only one percent in the early 2020s. Meanwhile, astrophysicists will likely continue to think about their ideas about the physics of the early universe.

The latest findings of the Riess team are published in the July 12 issue of the Astrophysical Journal .

Learn more:
The Hubble Enhanced Criterion Gives New Evidence of New Physics in the Universe