Accelerating expansion. How do we know this is true?

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In this article I will try to give you an idea about the process which revealed one of the most astonishing features of our Universe. It showed that the expansion rate of the Universe is accelerating.

Back in time it was thought that the expansion rate slows down by the presence of mass and nobody expected to find accelerating rate. However, this is what we’ve eventually got. Results obtained by Saul Perlmutter, Brian Schmidt and their teams are now considered to be an astronomical fact. It forced physicists to propose the idea of Dark Energy which is now widely accepted in astrophysical society.

So let’s have a quick look on how mentioned groups found out that our Universe expands with a growing rate.

You might have heard about various ways which are used to find the distance to far away objects. These are parallax method, Cepheid variables method and such. However, these two are confined in applicability by distance. That is, they are not applicable when the distance is very large (in astronomical terms). But there is another method which relies on another type of standard candles called supernovae type Ia. Such supernovae allow us to define the distance throughout the billions of light-years and are extremely useful. They were used by mentioned teams of astronomers in their observations. Let’s have a look on the process.

Firstly, I’ll try to give you an idea of what these supernovae actually are and how they occur. Soon after our Sun will run out of fuel its core will collapse and its outer layers will be thrown out to the interstellar space. Collapsed core will become a white dwarf star afterwards.

Collapse of a white dwarf stops when its electrons are compressed so strong that further squeeze is restricted by the so-called electron degeneracy which is the consequence of Pauli’s exclusion principle. Basically it means that two fermion particles (electrons in our case) can’t have the same quantum state or simplifying this we can say that two electrons can’t occupy the same position in space. Therefore, when the gravitational pull tries to squeeze the remaining core of a star electrons push against it providing additional pressure which prevents further collapse. Remaining white dwarf is very stable in this way and it can be kept in such a state for billions of years.

However, when a white dwarf has a close binary companion interesting things might occur. Because the white dwarf is so dense it would pull material from its companion and thus accumulate mass. Eventually, when the white dwarf gets the mass of roughly 1.4 solar masses the pressure given by electron degeneracy would not be enough to keep our white dwarf in a stable state. It would explode so violently that can easily overshine an entire galaxy. And because the explosion occurs with a certain mass (known as Chandrasekhar limit) all the stars which underwent it would have the same luminosity. There have recently been some articles doubting that such supernovae occur with Chandrasekhar mass and stating that most of them explode with mass below the limit. Nonetheless, their luminosity would be the same giving us a perfect tool for determining astronomical distances.

Now, when we have such an excellent standard candle we have to determine the expansion rate in different times. This can be done using Doppler shift method. The wavelength of light increases while it travels through the intergalactic space and by measuring the redshift we are able to determine the relative velocity of an object at the point of time when it emitted the light we observe today

Basically, two mentioned teams used the most powerful telescopes at the time to collect signals of distant supernovae from the wide range of galaxies. Eventually they got at least 4 dozen supernovae from a large variety of cosmic timescale, meaning that one supernova was found to explode 4 billion light years away (hence 4 billion years ago), another one – 8 billion light years away and so on. Then, taking unimaginably scrupulous calculations they came up with unforeseen results. The expansion rate of the Universe slowed down for the first 7 billion years after the Big Bang as was predicted, but it is no longer. When the Universe was 7 billion years old the expansion rate started to accelerate.

Most astronomers and physicists think that such acceleration is due to the Dark Energy which is still a hypothetical form of energy possessing the negative pressure, thus pushing objects apart from each other. The idea of such repulsive gravity was first proposed by Albert Einstein when he tried to reinstate his equations that they would depict static Universe. Static model was thought to be true at the time and when Alexander Friedman found the solution to Einstein’s equations saying that the Universe should either expand or contract, Einstein added the “Cosmological constant” into his equations, which would provide the negative pressure and keep the Universe static.

However, several years later Edwin Hubble showed that the Universe does actually expand and there is no need in Einstein’s cosmological constant. Einstein himself designated the cosmological constant as his “greatest blunder”. However, we now see how straight-forward thinking Albert Einstein was. His idea of cosmological constant comes back into play almost a century after emerging, though not in a form of constant but in a form of gradually changing entity of our spacetime.

Thank you.

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