Quasar measurements suggest that the universe is expanding faster than we thought



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The dark energy density seems to be increasing with time, and we are not sure why.

To infinity and beyond

Finding out that the universe is expanding has been one of the greatest turning points in astronomy and science in general. We do not know how great the universe is, nor do we know whether it is infinite or not, but we are sure that it is expanding. The first evidence emerged in the 1920s when Alexander Friedmann derived a set of equations known as Friedmann's equations, showing that the universe could expand. The theory really got windy a few years later, when Edwin Hubble discovered that some galaxies seem to be moving away from us.

Hubble also discovered that not only is the universe expanding, but its expansion is speeding up. That seemed impressive at the time. Not only is the universe getting bigger, but it's getting bigger, faster. Hubble calculated a universal expansion rate of 500 km / s / Megaparsec, with a megaparsec equivalent to 3.3 million light-years. So, every 3.3 million light-years away, the situation you are in is drifting 500 km faster – every second. Subsequent measurements have refined much and reduced this value, but there is still some controversy and uncertainty. Most studies, however, agree that the universal expansion rate is about 70 km / s / Megaparsec.

But it gets even weirder. Ironically, the universal expansion rate is also called the Hubble constant – when it is anything but constant. Not only different measures come up with slightly different values, but when you look at different parts of the universe, you will also find different rates of expansion.

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For example, the nearby universe, measured by telescopes like Hubble and Gaia, appears to have a value of 73 km / s / Mpc. Meanwhile, when the Planck telescope looked at the distant universe, it returned with a value of just under 70 km / s / Mpc. Therefore, the Hubble constant seems to vary both in time and space – both because it is a constant.

That is where the new study comes in – but, instead of clarifying things, it adds even more mystery.

The brightest of the brightest

Artistic representation of a distant quasar. Image credit: ESO / M. Kornmesser

Initial Hubble studies, like many subsequent measurements, were based on something called redshift. Essentially, as light travels from its original source to us, the space is stretched and the wavelength itself is stretched. This stretching changes the wavelength to the reddest parts of the spectrum – hence the name "redshift."

Now, if you want to look at something far away, you want a very powerful light source. In the new study, researchers focused on the brightest sources of light: quasars.

In a stellar twist of fate, the brightest light sources go hand in hand with the darkest objects in the universe: supermassive black holes. These black holes, believed to be at the center of all galaxies, are surrounded by a gaseous accretion disk. As the gas falls toward the black hole, the energy is released in the form of electromagnetic radiation with incredible power. This phenomenon is called a quasar, and some quasars are thousands of times brighter than the Milky Way, exactly what you want in this type of study. Quasars are scattered throughout the universe, making them the ideal target for multiple measurements.

Astronomers at the University of Durham in the United Kingdom and the Università degli Studi di Firenze in Italy used observations of 1,600 quasars to calculate the rate of expansion of the universe to about one billion years after its birth.

When you look at something that is a light-year away, you are essentially looking back and seeing this object as it was a year ago. In this case, astronomers analyze 12 billion years in the past. The strange thing is that the values ​​found for the expansion rates of 12 billion years ago were similar to the rates of expansion reported by previous studies that looked at areas around 8 billion years ago. In other words, two different epochs had the same rate of expansion, when in fact they should not. There is nothing in our current arsenal of cosmological knowledge that can explain this convincingly.

"When we combine the quasar sample, which spans nearly 12 billion years of cosmic history, with the most local sample of type Ia supernovae covering only the last eight billion years, we find similar results in overlapping times," said Dr. Elisabeta Lusso of Durham University said in a statement.

"However, in previous phases, which we can only plumb with quasars, we find a discrepancy between the observed evolution of the Universe and what we could predict based on the standard cosmological model."

Of course, this is still an initial study, which will be thoroughly investigated and replicated – but if it is confirmed, then astrophysicists will have a lot of research to find an explanation.

However, a possible solution, still speculative at this time. could have something to do with the indescribable dark energy, a theoretical form of energy postulated to act in opposition to gravity and occupy the entire universe. The lead author, Dr. Guido Risaliti, from the Università degli Studi di Firenze, concludes:

"One of the possible solutions to the expansion of the primordial universe would be to invoke evolving dark energy with a density that increases over time.

The study "Cosmological constraints of the Hubble quasars diagram in high redshifts" was published in Astronomy of Nature.

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