Recently, the "Hubble tension" has emerged in cosmology, characterized by conflicting measurements of expansion rates, which has raised questions about standard cosmological models. A new theory suggests that a vast, under-dense void could explain the differences, challenging conventional views of the distribution of matter in the universe and suggesting a potential revision to Einstein's theory of gravity.
Cosmologists have proposed a giant void in space as a solution to the "Hubble Tension," challenging traditional models and suggesting revisions to Einstein's theory of gravity.
One of the biggest mysteries in cosmology is the rate at which the universe is expanding. This is predicted by the Standard Model of Cosmology, also known as Lambda-cold dark matter (ΛCDM). The model is based on detailed observations of the light left over from the Big Bang, known as the cosmic microwave background (CMB).
The expansion of the universe drives galaxies farther away from each other. Galaxies farther away from us move faster. The relationship between a galaxy's speed and distance is governed by the "Hubble constant", which is approximately 43 miles (70 kilometers) per second per megaparsecond (a unit of length in astronomy). This means that for every million light-years a galaxy moves away from us, its speed increases by about 50,000 miles per hour.
Unfortunately for the Standard Model, this value has recently been disputed, leading to what scientists call the "Hubble tension." When we measured the expansion rate using nearby galaxies and supernovae (exploding stars), it was 10% greater than what we predicted based on the CMB.
In our new paper, we propose one possible explanation: We live in a vast void of space (a region with a lower-than-average density). Our study shows that this may inflate local measurements through outflows of material from the void. Outflows occur when denser regions around the void pull it apart - they exert a greater gravitational pull than the less dense material inside the void.
In this case, we need to be close to the center of a void with a radius of about a billion light-years, which is about 20% less dense than the average density of the entire universe - so it is not completely empty.
Such a huge and deep void was unexpected in the Standard Model and has therefore been controversial. The CMB provides a snapshot of the structure of the early universe, suggesting that matter should be evenly distributed today. However, directly counting the number of galaxies in different regions does suggest that we are in the middle of a local void.
Adjust the law of gravity
We wanted to test this idea further by assuming that we live in a large void that was formed by small early density fluctuations, matching many different cosmological observations.
To this end, our model does not employ ΛCDM but an alternative theory called Modified Newtonian Dynamics (MOND).
MOND was originally proposed to explain the anomalies in the rotation speed of galaxies. This is also the reason why people proposed "dark matter", an invisible substance. MOND believes that Newton's law of universal gravitation will fail when the gravity is very weak, which is the case in the outer regions of galaxies.
The overall expansion history of the universe in MOND is similar to the Standard Model, but structures (such as galaxy clusters) grow faster in MOND. Our model captures what the local universe might look like in the MOND universe. We also found that it allows today's local expansion rate measurements to fluctuate depending on our location.
Recent observations of the galaxy provide an important new test of our model based on its predicted velocities at different locations. This can be achieved by measuring a value called "bulkflow". "Volume flow" is the average velocity of matter in a given sphere, whether dense or not. It varies with the radius of the sphere, and recent observations suggest that its radius can extend to a billion light-years.
Interestingly, the overall flow of galaxies at this scale is four times faster than expected by the Standard Model. It also appears to increase with the size of the area considered - contrary to the predictions of the standard model. The chance that this is consistent with the Standard Model is less than one in a million.
This prompted us to look at how our study predicts body flow. We find that it agrees very well with the observations. This requires us to be quite close to the center of the void, and the center of the void is the emptiest.
Case closed?
Our results come at a time when popular solutions to the Hubble tension are in trouble. Some people think we just need more precise measurements. Others think this problem can be solved by assuming that the high expansion rates we measure locally are actually the correct expansion rates. But this requires a slight adjustment to the expansion history of the early universe so that the CMB still looks correct.
Unfortunately, an influential review highlighted seven problems with this approach. If the expansion of the universe had been 10% faster for most of its history, the universe would also have been 10% younger -- contradicting the ages of the oldest stars.
The presence of local holes of depth and breadth in galaxy population counts, as well as the observed fast volume flows, strongly suggest that structure is growing faster than expected in ΛCDM on scales of tens to hundreds of millions of light-years.
Interestingly, we know that the massive galaxy cluster El Gordo (pictured above) formed too early in the history of the universe and has too high a mass and collision rate to fit the Standard Model. This is further evidence that structure formation is too slow in this model.
Because gravity is such a dominant force on large scales, we will likely need to extend Einstein's theory of gravity—general relativity—but only to scales greater than a million light-years.
However, we don't have a good way to measure how gravity behaves on larger scales -- there aren't such large gravitationally bound objects. We can assume that general relativity is still valid and compare it with observations, but it is precisely this approach that has led to the very serious contradictions that our best cosmological models currently face.
Einstein is said to have said that we cannot solve problems by thinking the same way that caused them. Even if the changes required are not earth-shattering, we may be seeing the first reliable evidence in more than a century that we need to change our theory of gravity.
Author: IndranilBanik, Postdoctoral Researcher in Astrophysics at the University of St. Andrews.