A large discrepancy between different measurements of the expansion rate of our universe could be explained if our galaxy, the Milky Way, is located in a void of two billion light years. That is the conclusion of scientists who claim that a modified theory exists gravity can replace the standard model of cosmology. However, this hypothesis is strongly disputed by many astronomers.
The standard model by cosmology describes how we live in a universe dominated by dark energy And dark matter. Dark energy is a mysterious force that is apparently the cause of… expansion of the universe to accelerate, while dark matter provides most of the universe’s gravity and is thought to surround galaxies in halo-like shapes and prevent them from breaking apart. Together, these elusive phenomena describe how matter is distributed across the cosmos and how galaxies move relative to each other.
However, one of the biggest challenges that the Standard Model of cosmology must overcome is known as the ‘Hubble tension’. This concept is not named after the space telescope as you might imagine, but rather its namesake, astronomer Edwin Hubble. In 1929, Edwin Hubble discovered that the further away 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 later became known as the Hubble-Lemaître law (after the Belgian theoretical physicist and priest Georges Lemaître, who also discovered it independently). It says that the speed at which a galaxy is moving away from us is a product of its distance multiplied by the expansion rate of the universe, which is given by a parameter called Hubble’s constant.
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Since the time of Edwin Hubble, astronomers have strived to measure the Hubble constant with increasing accuracy. By knowing Hubble’s constant, and therefore knowing exactly how fast the universe is expanding, we can calculate how old the universe must be before it reaches its current size. Our current best measurements give the 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 red-shifted light of type Ia supernovas have resulted in a Hubble constant value of 73.2 kilometers per second per megaparsec. In other words, it says that each volume of space is a megaparsec in diameter (a parsec is 3.26 light years, and a megaparsec is a million parsecs, so 3.26 million light years) is expanding by 73.2 kilometers (45.5 miles) every second.
Yet the expansion rate of the universe is also ingrained in the physics of the universe cosmic microwave background (CMB) radiation. Measurements of the CMB by the European Space AgencyPlanck’s Planck mission gives a value of the Hubble constant of 67.4 kilometers per second per megaparsec. Both measurements were performed with high accuracy, but they cannot both be correct.
This strange dichotomy, which has become known as the Hubble tension, is now perhaps the most vexing problem in cosmology. While some astronomers suspect this is due to a measurement error somewhere along the line, others think it could indicate new physics.
That’s exactly what a new paper from scientists from Germany, Scotland and the Czech Republic proposes.
“The universe appears to be expanding faster in our environment – that is, to a distance of about three billion light years – than in its entirety,” said one of the paper’s authors, Pavel Kroupa of the University of Bonn in Germany. in a press statement. “And that really shouldn’t be the case.”
Their hypothesis centers on an astrophysical oddity called the Keenan-Barger-Cowie supervoid, named after the trio of astronomers who studied it. The supervoid is a so-called ‘under-density’ of matter in the universe, an area where there are statistically fewer galaxies on average – and our galaxy happens to be right in the middle of it all, say the scientists.
Outside this supervoid, galaxies are on average slightly more densely packed, resulting in more gravity that can pull objects in the supervoid toward it. This could give the impression that room is expanding faster in our environment, the team suggests, because galaxies are being dragged by the gravitational pull of matter beyond the supervoid.
“That’s why they’re moving away from us faster than would actually be expected,” said co-author Indranil Banik of the University of St. Andrews in Scotland.
The Standard Model of cosmology says that matter should be distributed fairly evenly throughout the universe, and that any voids should not grow beyond a certain size. Therefore, it has some difficulty in explaining a supervoid as large as the Keenan-Barger-Cowie void. Some astronomers, including Kroupa and Banik, believe that the Standard Model cannot explain this, while others, such as Martin Sahlén, Iñigo Zubeldía and Joseph Silk of the University of Oxford, have done so. went on the record say it is possible.
In the hypothesis of Kroupa, Banik and their co-author (Sergij Mazuurko from the Universität Bonn and Moritz Haslbauer from Charles University in the Czech Republic), our current theory of gravity, and therefore of dark matter, is replaced by a new theory called Modified Newtonian Dynamics, or MOND for short. This states that gravity behaves differently at low accelerations than described Einstein and Newton, and that the extra gravity can replace the need for dark matter. In the MOND paradigm, the universe could more easily create large voids, such as the Keenan-Barger-Cowie supervoid.
However, the idea that the presence of the void can affect measurements of the universe’s expansion rate has been hotly contested in the past. Nobel Prize winner Adam Riess of Johns Hopkins University in Baltimore, who, together with W. D’Arcy Kenworthy of Johns Hopkin and Dan Scolnic of Duke University in the United States, is leading efforts to measure the Hubble constant with supernovae from the United States type Ia, has shown that these type Ia supernovae have been observed outside the boundary of the supervoid had the same expansion rate like those in the void. In response, Kroupa, Banik, Mazuurko, and Haslbauer argue that the effect of the supervoid would be felt far beyond the void itself, and thus one would expect to measure a higher expansion rate in supernovae beyond the boundaries of the void.
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Other methods of measuring the Hubble constant, which are independent of the supervoid and the Standard Model of cosmology, also argue that the Hubble tension cannot be explained away. By tracking the angular distance in the sky that water massages into molecular clouds orbiting Earth supermassive black holes in distant galaxies, and inferring their physical distance from the geometry, has yielded a value of the Hubble constant of 73.9 kilometers per second per megaparsec, which is close to the Type Ia supernova measurements, given the uncertainty in the maser measurements . There is also the H0LiCOW (H0 refers to the Hubble constant) project, which investigates how light from quasars in the early universe, several paths of different lengths may pass through the foreground gravity lenses. Quasars often have fluctuations in their brightness; As the universe moves through the different paths through the gravitational lens, the universe is still expanding, and the rate of this expansion is imprinted on the different lens images of the quasar’s brightness variations. This project finds that the expansion rate is 73.3 kilometers per second per megaparsec, almost identical to the value of a Type Ia supernova.
These measurements conflict with the CMB measurement and are unrelated to the hypothesis that the supervoid can create the Hubble strain. So if that hypothesis is to ultimately gain traction, it looks like Kroupa, Banik, Mazuurko and Haslbauer will have to convince a lot more people.
The hypothesis was published in the journal in November Monthly notices of the Royal Astronomical Society.
For all readers interested in further reading, a list of articles on the topic of the Hubble voltage and their measured values of the Hubble constant can be found. here.