The topic about the age and the expansion rate of the Universe has lately become the main subject of a number of discussions among the scientific society using a wide variety of astronomical technologies - different telescopes and techniques. 

Now there is a new participant from the desert in Chile - the Atacama Cosmology Telescope. As a result of its study of the “oldest light” in the sky it has come to a conclusion that the so-called Big Bang happened 13,77 billion years ago, that is approximately 40 million years. This is really close to the number that the European Space Agency’s (ESA) observatory Planck got in the early 2010s (it operated only from 2009 to 2013). At the beginning of February 2010 Planck’s mission had started the all-sky survey that managed to map successfully the ancient light. But the big problem comes with the fact that lots of other telescopes using various methods, that slightly differ from these, found out data showing that the same light appears to be a few hundred million years younger.

They all are trying to measure the value known as the Hubble Constant - this is the unit used for describing “how fast the universe is expanding at different distances from a particular point in space”. According to Hubble’s law, the further the galaxies are the faster they are moving away from Earth. This observation is the first basis for the “expansion of the universe” and is considered to be one of the main evidences supporting the Big Bang theory. It was first described in detail by Edwin Hubble, an American astronomer, in 1929 and ever since every universe researcher is trying to measure it. 

There are two approaches to this discovery. The first one is to observe the state of the universe just after the Big Bang happened and to use the known physics at this time of the 20th century to try to predict the constant. The other one is to depict the distance between a local variable (cepheids - this is a type of star that pulsates radially, varying in both diameter and temperature) and exploding stars (supernovas - in fact a powerful and luminous stellar explosion) and to try to measure precisely the recessional rate (basically the rate at which the astronomical object become more distant from the observer). 

The Atacama Cosmology Telescope (ACT) and Planck have both pursued the first concept, because of that they needed to survey the Cosmic Microwave Background (CMB) firstly. The CMB is the oldest electromagnetic radiation in the universe, it represents faint cosmic background radiation filling the space. Once the Universe had cooled enough, the electrons and protons in the cosmos combined and led to a formation of neutral hydrogen atoms. The first light that “sweeps out across space” is still glowing above the Earth at microwave frequencies and its temperature is really close to the absolute zero, just 2,7 degrees above it. This signal can be detected with telescopes but with certain time deviations in the signal, even though the astronauts can still find out a lot about the Big Bang and the creation of the universe from the information it carries. One of the values that can be extracted is the mentioned Hubble Constant. 

Earlier this month the international team working with the Atacama Cosmology Telescope published just a review of the data received. According to it the Universe has expanded with 67,6 km per second for every 3,26 million light-years. The result from Planck’s mission was 67,5 km per second. The results are very similar and many people were not surprised by it, because they used almost the same approach. However,  these were not the same experiments as Prof Erminia Calabrese, working with ACT, explains. She added that Planck actually went to space but the ACT stayed on the ground and when on the Earth a smaller angular scale is observed than in the cosmos and it is not always the same outcome. There could have been different results.

In contrast, there have been another Hubble Constant produced by the Hubble Space Telescope and the Gaia space observatory that is equal to 74km per second per megaparsec (3.26 million light-years). They measured this with the alternative method above mentioned. 

Both sides have completely different results which might mean that one of them or even both are wrong or that there is something fundamental out there that humans still do not understand.