by A large discrepancy between different measurements of the expansion rate of our universe could be explained if our galaxy, the Milky Way, sits in a vacuum of two billion light years. That is the conclusion of scientists who argue that the theory is modified gravity instead of the standard model of cosmology. However, this hypothesis is highly disputed by many astronomers.
The IS standard model of cosmology describe how we live in a dominated universe dark energy and dark dark. Dark energy is a mysterious force that is creating the expansion of the universe to accelerate, although dark matter provides most of the gravity in the universe and is thought to surround galaxies in halo-like shapes and prevent them from collapsing. Together, these invisible phenomena describe how matter is distributed throughout the cosmos and how galaxies move relative to each other.
One of the biggest challenges facing the standard model of cosmology, however, is called the “Hubble Tension.” This concept is not named for space telescope as you might imagine, but in his name, an astrologer Edwin Hubble. In 1929, Edwin Hubble discovered that the more distant a galaxy is, the faster it appears to be moving away from us. He was able to derive a relationship to describe this connection, which was called the Hubble – Lemaître’s Law (after the Belgian physicist and theoretical priest, Georges Lemaître, who also discovered it independently). It states that the velocity with which a galaxy is moving away from us is the product of its distance multiplied by the expansion rate of the universe, which is given by a parameter known as the Hubble constant.
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Since Edwin Hubble’s day, astronomers have strived to measure the Hubble constant with greater precision than ever before. By knowing the Hubble constant, and therefore exactly how fast the universe is expanding, we can calculate how old the universe must be for it to have reached its current size. Our current best measurements are put on the age of the universe at 13.8 billion years.
However, there is a problem.
Measurements of the expansion of the universe created by measuring the redshifted light of type Ia supernova resulting in a Hubble constant value of 73.2 kilometers per second per megavarsec. In other words, it says that every volume of space is a megaparsec across (a parsec It’s 3.26 light years, and a megaparsec is a million parsecs, so 3.26 million light years) expanding 73.2 kilometers (45.5 miles) every second.
However, the expansion rate of the universe is baked into the physics of the cosmic microwave background (CMB) radiation. Measure the CMB at the European Space AgencyThe Planck mission gives the Hubble constant value of 67.4 kilometers per second per megasecond. Both measurements are made to a high degree of accuracy, but neither measurement can be correct.
This strange dichotomy, now known as the Hubble Tension, is arguably the worst problem in cosmology. While some astronomers suspect it’s a measurement error somewhere along the line, others think it could be signaling new physics.
That’s exactly what a new paper suggests, from scientists in Germany, Scotland and the Czech Republic.
“The universe … seems to be expanding faster in our neighborhood – that is, up to a distance of about three billion light years – than as a whole,” says one of the authors of the paper, Pavel Kroupa from the University of Bonn in Germany, i press release. “And that really shouldn’t be the case.”
Her hypothesis focuses on an astrophysical oddity known as the Keenan-Barger-Cowie supervoid, named after the three astronomers who studied it. The supervoid is an “underdensity” of matter in the universe, a region where there are statistically fewer galaxies on average — and our The Milky Way Galaxy which just happens to be sitting right in the middle, say the scientists.
Outside of this supervoid, galaxies are on average a little more densely packed, resulting in more gravity that can pull objects within the supervoid towards them. This could suggest that space growing faster in our vicinity, the team suggests, as galaxies are pulled along by the gravity of matter outside the vacuum.
“That’s why they’re moving away from us faster than actually expected,” said co-author Indranil Banik of the University of St Andrews in Scotland.
The standard model of cosmology states that matter should be spread evenly throughout the universe, and that no void should grow beyond a certain size. Thus, he has some difficulty explaining a supervacuum as large as the Keenan-Barger-Cowie vacuum. Some astronomers, including Kroupa and Banik, believe that the standard model cannot explain it, while others such as Martin Sahlén, Iñigo Zubeldía and Joseph Silk of the University of Oxford gone on record saying yes.
In the hypothesis of Kroupa, Banik and their co-authors (Sergij Mazurenko of Universität Bonn and Moritz Haslbauer of Charles University in the Czech Republic), our current theory of gravity, and therefore dark matter, is replaced by a new theory called Modified Newtonian Dynamics. . or MOND for short. This proves that gravity behaves at low accelerations as described Einstein and Newton, and that the extra gravity can replace the need for dark matter. In the MOND paradigm, the universe could most easily form large voids such as the Keenan–Barger–Cowie supervacuum.
However, the idea that the presence of a vacuum can affect the measurement of the expansion rate of the universe has been controversial in the past. Nobel laureate Adam Riess from Johns Hopkins University in Baltimore, who is leading efforts to measure the Hubble constant with type Ia supernovae, together with W. D’Arcy Kenworthy from Johns Hopkins and Dan Scolnic from Duke University in the United States, showed that kind. Ia supernovae observed outside the boundary of the mass vacuum the rate of expansion was the same as those inside the void. In response, Kroupa, Banik, Mazurenko and Haslbauer argue that the effect of the supervacuum would be felt far beyond the void itself, so a higher rate of expansion in supernovae would be expected to be measured beyond the boundaries of the vacuum.
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Other methods of measuring the Hubble constant, independent of the super-vacuum and the standard model of cosmology, say that the Hubble Tension cannot be explained away. By tracking the angular distance on the sky that water masers in molecular clouds orbit supermassive black holes in distant galaxies, and then deriving their physical distance from the geometry, yields a Hubble constant value of 73.9 kilometers per second per megaparsec, which is close to the measurement of a type Ia supernova, given the uncertainty in the maser measurements. The IS H0LiCOW (H0 refers to the constant Hubble project), which studies how light comes from quasar early in the Universe can take different paths of different lengths through the foreground gravitational lenses. Quasars often have fluctuations in their brightness; while traversing the different paths through the gravitational lens, the universe is still expanding and the rate of this expansion is imprinted on the different lensed images of the quasar brightness variations. This project finds that the expansion rate is 73.3 kilometers per second per megasecond, almost equal to the value of a type Ia supernova.
These measurements are in conflict with the CMB measurement, and are independent of the hypothesis that the supervoid can create the Hubble tension. So ultimately, if that hypothesis is to have legs, it looks like Kroupa, Banik, Mazurenko and Haslbauer will have to convince a lot more people.
The hypothesis was published in November in the journal Monthly Notices of the Royal Astronomical Society.
For any readers interested in further reading, a list of papers on the subject of the Hubble Tension and their measured values of the Hubble constant can be found. here.
