The next time you eat a berry muffin, think about what happened to the blueberries in the dough as the sweetness baked. The blueberries lay in one place, but as the bun expanded, the berries began to move away from each other. If you could stand on one berry, you would see everyone else move away from you, but the same will be true for any other berry you choose. In this sense, galaxies are like berries in a cupcake.
Since the Big Bang, the universe has been expanding relentlessly. The strange fact is that there is no single place from which the universe is expanding - rather, all galaxies are (on average) moving away from others. From our vantage point in the Milky Way galaxy, it will appear that most galaxies are moving away from us - as if we are the center of our bun-like universe. But look from any other galaxy and the view will be exactly the same.
To further confuse you, new research suggests that the rate at which the universe expands can be different depending on how far back in time you look. New data, published in the Astrophysical Journal, indicate that it is time to rethink our understanding of space.
Hubble mystery
Cosmologists characterize the expansion of the universe by a simple law - the Hubble law (named after Edwin Hubble). Hubble's Law is the observation that more distant galaxies move away faster. This means that nearby galaxies move relatively slowly.
The relationship between speed and distance to the galaxy is determined by the "Hubble constant" - 70 km / s / Mpc. This means that the galaxy moves about 90,000 km per hour for every million light years away from us.
This expansion of the universe, with nearby galaxies moving away more slowly than distant galaxies, is expected from a uniformly expanding space with dark energy (an invisible force that accelerates the expansion of the universe) and dark matter (an unknown and invisible form of matter, which is five times larger than usual). The same can be observed in a muffin with berries.
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The history of measuring the Hubble constant is full of difficulties and unexpected revelations. In 1929, Hubble himself believed that its value should be on the order of 600,000 km per hour per million light years - about ten times more than is currently measured. Attempts to accurately measure the Hubble constant over the years have led to the inadvertent discovery of dark energy. Finding information about this mysterious type of energy, which accounts for 70% of the energy in the universe, inspired the launch of the best space telescope in the world (to date), named after Hubble.
The catch is that the results of the two most accurate measurements do not agree and do not correlate with each other. Once the cosmological measurements became so accurate that they showed the value of the Hubble constant, it became obvious that this did not make sense. Instead of one, we have two conflicting results.
On the one hand, we have new precise measurements of the cosmic microwave background - the Big Bang afterglow - made by the Planck mission, which measured the Hubble constant as 67.4 km / s / Mpc.
On the other hand, we have new measurements of pulsating stars in nearby galaxies, also incredibly accurate, which measured the Hubble constant as 73.4 km / s / Mpc. They are closer to us in time.
Both of these measurements claim to be correct and very accurate. The discrepancy between the measurements is about 500 km per hour per million light years, so cosmologists call it "tension" between two dimensions - they sort of stretch the statistics in different directions, and it must collapse somewhere.
New physics?
How will it collapse? Nobody knows at the moment. Perhaps our cosmological model is wrong. It can be seen that the universe is expanding faster closer to us than we would expect, starting from more distant dimensions. Measurements of the cosmic microwave background do not measure local expansion, but do it through a model - our cosmological model. She has been extremely successful in predicting and describing many of the observable data in the universe.
Therefore, although this model may be wrong, no one has come up with a simple convincing model that can explain both this and everything that we observe. For example, we could try to explain this with a new theory of gravity, but then other observations do not fit. Or it could be explained by the new theory of dark matter or dark energy, but then other observations will not work - and so on. Therefore, if this "tension" is associated with new physics, it must be complex and unknown.
A less interesting explanation would be “unknown unknowns” in the data caused by systematic effects, and closer analysis will one day reveal a subtle effect that has been missed. Or it might just be a statistical fluke that will disappear when more data is collected.
It is currently unclear what combination of new physics, systematic effects, or new data will resolve these tensions, but something will surely become clear. The picture of the universe as an expanding cake may be wrong, and cosmologists are challenged to come up with a different picture. If new physics is needed to explain the new dimensions, then the result will change our understanding of space.
Ilya Khel