# How did we find out the age of the universe and is it really important

Even two decades ago, scientists assumed that the age of the Universe was in the range from seven to twenty billion years, but today, thanks to a series of great discoveries and improvements in research methods, we conclude with accuracy that the Universe is 13.8 billion years old. How exactly did we figure this out and does it make any sense?

Video on the discovery of dark energy and the accelerated expansion of the universe.
My telegram channel about cosmology: t.me/iluniverse.

It should be said that if the Universe has a certain period of existence, then it once arose. And it arose from a small dense point, which began to expand. The fact of the expansion of the Universe was recorded by Edwin Hubble in 1929 thanks to the observation of a uniform recession of galaxies. Hubble concluded that objects that are far away from us run away from us faster than those that are in close proximity to the Milky Way. Thus, the Hubble law was formulated, which was later explained by the Big Bang theory – the Universe expands like an inflating ball, therefore, the objects inside it scatter from each other the faster the farther they are. Previously, it was assumed that the Universe is non-dynamic and eternal.

If all galaxies were once at the same point, and then ran away from each other, that is, there was a Big Bang, then just by the rate of expansion, you can determine the moment when they began to do this. This can be represented in a simple model: imagine two cars moving away from each other at a common constant speed. Knowing the position of the cars relative to each other and the speed of their movement, we can determine the time when the cars began to disperse.

And how to estimate the speed of recession of galaxies? According to Hubble’s law. Then the time since the Big Bang will be inversely proportional to the rate of expansion, that is, the Hubble parameter. The discoverer himself determined the value of the parameter to be 500 km/s/Mpc. That is, a galaxy located at a distance of 3.2 million light years would be moving away from us at a speed of 500 km / s. Then the age of the Universe was estimated at 2 billion years. But the catch was that at the beginning of the 20th century, the age of the Earth was already measured quite accurately and was estimated at 4 billion years. This meant that the rate of expansion was calculated incorrectly. By the 1990s, scientists had narrowed down the range of possible values ​​to 50-90 km/s/Mpc, which would have given the universe an age of 7-20 billion years, which was still a very rough estimate. The main reason for this was the lack of an object that allows you to explicitly determine the rate of expansion.

To accurately estimate the rate of expansion, two parameters must be known: the rate of removal of the object and the distance to it. The removal rate is determined by the shift of the spectrum lines – the longer the photon travels through the expanding Universe, the more its wave is stretched. But the distance to the object is more difficult to determine, and this should be shown with an example. Let’s take two light bulbs of the same power and require them to be placed at different distances. Let the first light bulb be hung two meters from us, and the second – further away, but it is not known how much. Let’s find out at what distance it is: to do this, we will determine its luminosity and compare it with the luminosity of the first light bulb. Let the luminosity of the second bulb be four times less than that of the first. Then, according to the inverse square law, it will not be difficult to conclude that the second bulb is twice as far as the first, that is, it is located four meters from us. If you find such bulbs in the Universe and estimate the distance to at least one of them, then you can find out where the rest of the bulbs are.

The problem is that there are no objects of the same brightness in the Universe … At least, we thought so. But it turned out that such objects still exist, even if they appear very rarely and for a very short time – these are type Ia supernovae. They arise from a binary system when one of the regular stars turns into a red giant and transfers its shell to the second star. Then only a dense nucleolus remains from the first star – a white dwarf, and the second star itself turns into a red giant. But the carousel continues, and the red giant again sheds its shell, but this time onto a white dwarf. This dense object has a certain mass limit, after which the dwarf cannot exist – this is the Chandrasekhar limit, equal to 1.44 solar masses. And it happens that the dwarf is saturated with matter so much that it overcomes this threshold, after which an explosion occurs, from which nothing but a gas and dust cloud remains. In the Universe, this event is reflected by the brightest flash – the brightest for a point object.

We were able to conclude that Type Ia supernovae always burst with the same energy. Fixing SN Ia turns into collecting identical light bulbs, counting the distance to which we estimate the rate of expansion of the Universe and its age. It is thanks to the discovery of SN Ia as the so-called. standard candle in 1998, it was possible to determine that the Universe is expanding not with deceleration, as it was logically assumed for almost 70 years, but with acceleration, and it was thanks to this that we were convinced of the existence of dark energy. I talk about the discovery process and about type Ia supernovae in my recent video.

In 1999, according to the photometric data of SN Ia, the Hubble parameter was estimated at 71 km/s/Mpc, which corresponds to the age of the Universe at 12 billion years. More accurate measurements of the expansion rate are made not by standard candles, but by the inhomogeneities of the relic microwave background (Planck 2018 results). Thanks to them, we came to the conclusion that the Universe is a little less than 14 billion years old.

By itself, the estimate of the age of the universe does not give us anything. Age is a consequence of other more important observations, and its significance only adorns the headlines. Nevertheless, there is reason to think whether the Universe is young or not. How do you think?

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