# Where is the cosmos going?

Although we do not know this, we can already determine with a high degree of accuracy how fast it is speeding up. That is, at what speed the galaxies, the basic material structures of the Universe, are moving away from each other.

The sequence of events is as follows: when, in 1929, Edwin Hubble found that galaxies — whose existence he himself had discovered five years earlier — were moving away from each other faster and faster as their distance increased, the modern science of the Universe was born. Its starting point and foundation was the concept of the Big Bang, which gave the Universe the dynamics that allowed it to expand.

A derivative of this discovery was the establishment of the so-called Hubble constant. It says that the Universe is expanding, and therefore every megaparsec (Mpc) of space grows by a certain value every second (a megaparsec is one million parsecs, and a parsec — pc — is 3.26 light years). In other words — the Hubble constant describes the rate of expansion of the Universe as a function of time, and allows us to calculate how much a galaxy that is a megaparsec (3.3 million light years) away from us moves away every second. However, the exact calculation of this “how much” is not at all an easy task.

Initially, this constant was set very high, at a few hundred kilometers per second in a megaparsec — this result was obtained with the Hubble Space Telescope in the 1990s. Later, the value was found to be significantly lower — that is, a few tens of kilometers per second. The Hubble Space Telescope — [Photo: European Space Agency, CC BY 4.0, via Wikimedia Commons]

# The Hubble constant thanks to observations of Cepheids and the Planck probe

With time, already in our century, astronomers began to use more advanced methods to determine the Hubble constant, which is the basic parameter of observational astronomy, thanks to which we can determine the age, scale and size of the Universe around us.

There are several such methods. Two basic ones are counting the constant thanks to observations of so called cepheids, i.e. pulsating variable stars, which we discover outside our Milky Way, e.g. in the Large Magellanic…