**：pablo carlos budassi/wikipedia, cc by-sa
One of the biggest mysteries in cosmology is the rate at which the universe is expanding. This can be done using the Standard Model of cosmology, also known as Cold Dark Matter (CDM). This model is based on detailed observations of the light left over from the big **, the so-called cosmic microwave background (CMB).
The expansion of the universe keeps galaxies away from each other. The farther away they are from us, the faster they move. The relationship between the speed and distance of galaxies is determined by the Hubble constant, which is about 43 miles (70 kilometers) per second per million parsecs (a unit of length in astronomy). This means that a galaxy is growing at a rate of 50,000 miles per hour every million light-years away from us.
But unfortunately, for the Standard Model, this value has recently been controversial, leading to what scientists call the Hubble Tension. When we measure the expansion rate using nearby galaxies and supernovae (the stars of **),it is 10% larger than our expansion rate according to CMB**.
In our new **, published in the Monthly Notices of the Royal Astronomical Society, we propose a possible explanation: we live in a huge void in space (an area with below-average density). We show that this may be measured by the expansion of matter flowing out of the void. Effluents are created when the denser area around the vacuum pulls it apart – they exert a greater gravitational pull than the less dense matter inside the vacuum.
In this case, we need to be close to the center of a hole with a radius of about 1 billion light-years, which is about 20% less dense than the average density of the entire universe – so not completely empty.
Such large and deep cavities are unexpected, and therefore highly controversial, in the Standard Model. The CMB gives a snapshot of the structure of the universe in infancy, showing that matter today should be evenly distributed. However, directly counting the number of galaxies in different regions does show that we are in a local void.
Change the law of gravitation
We wanted to further test this idea by matching many different cosmological observations to assume that we live in a huge space that was formed by early small density fluctuations.
In order to do this, our model does not use CDM, but instead uses another theory called modified Newtonian dynamics (MOND).
Mond was originally proposed to explain the anomaly in the speed of galaxies' rotation, which led to the existence of an invisible substance known as "dark matter". Instead, Mond argues that this anomaly can be explained by Newton's law of gravitation, which invalidates when gravity is very weak – as is the case in the outer regions of galaxies.
In general relativity, the entire universe expansion history is similar to the Standard Model, but structures, such as galaxy clusters, grow faster in general relativity. Our model captures what the local universe looks like in the MOND universe. We've found that it allows today's local expansion rate measurements to fluctuate depending on where we are.
Recent galactic observations have given our model a key new test, based on its velocity at different locations. This can be achieved by measuring the so-called volumetric flow, which is the average velocity of a substance in a given sphere, regardless of density. This varies with the radius of the sphere, and recent observations suggest that it lasts up to 1 billion light-years.
Interestingly, at this scale, the overall flow velocity of galaxies is four times that expected in the Standard Model. It also seems to increase with the size of the area being considered – which is the opposite of the Standard Model. This is less than a 1 in a million probability of alignment with the Standard Model.
CMB temperature fluctuations (color aberration). *NASA.
This led us to look at our study of bulk flows. We found it to be in good agreement with the observations. This requires us to be fairly close to the center of the hollow, which is the emptiest.
The truth is revealed?
Our results come at a time when Hubble Tension's popular solution is in trouble. Some people think we just need more precise measurements. Others argue that this can be solved by assuming that the high expansion rates we measure locally are actually correct. But this requires a slight tweak to the expansion history of the early universe so that the CMB still looks correct.
Unfortunately, an influential review highlights seven problems with this approach. If the universe expands 10% faster for most of the universe's history, it will also be 10% younger – contradicting the age of the oldest stars.
The presence of a deep and extended local hole in the galaxy, the large flow counted and quickly observed strongly suggests that the CDM structure is growing faster than expected on the scale of tens to hundreds of millions of light-years.
Interestingly, we know that the massive galaxy cluster El Gordo formed too early in the history of the universe and had too high a mass and collision velocity to be incompatible with the Standard Model. This is further evidence that in this model, structure formation is too slow.
Since gravity is the dominant force on such a large scale, we may well need to extend Einstein's theory of gravity – the general theory of relativity – but only on a scale of more than a million light-years.
However, we don't have a good way to measure the behavior of gravity on a larger scale – there is not that much gravity to bind objects. We can assume that general relativity is still valid and compare it with observations, but it is this approach that leads to the very serious contradictions that our best cosmological models currently face.
It is believed that Albert Einstein once said that we cannot solve problems with the thinking that caused them in the first place. Even if the desired change is not drastic, we are likely to witness the first credible evidence in more than a century that we need to change our theory of gravity.