How fast is the universe expanding?

The Hubble and Gaia teams have joined together to make the most accurate measurement for today.

In the 1920s, Edwin Hubble made a revolutionary discovery - it turned out that the Universe was expanding. Initially, this situation was predicted by Einstein’s General Theory of Relativity. The speed of this expansion is called the Hubble constant . To date, with the help of modern telescopes - such as the Hubble telescope - astronomers have re-measured and revised this value many times already.

These measurements confirmed that the expansion rate increased over time, although scientists are not sure why. The last measurements were carried out by an international team of scientists who used data from Hubble, and then compared them with data obtained at the Gaia Observatory.European Space Agency. As a result, the most accurate measurements of the Hubble constant to date were obtained, which, however, did not remove questions about the cosmic acceleration.

A study describing these discoveries was published in July in the Astrophysical Journal entitled: “The Milky Way Cepheid Standards for Measuring Space Distances and Their Application to Gaia DR2: Implications for the Hubble Constant”. The study involved scientists from the Space Research Institute using a space telescope, Johns Hopkins University, the National Institute of Astrophysics, the University of California at Berkeley, the University of Texas A & M, and the European Southern Observatory. Three steps to measure the Hubble constant: measuring parallax for Cepheids, measuring galaxies containing Cepheids and type Ia supernovae, measuring distant galaxies containing type Ia supernovae. Since 2005, Adam Riess




- Nobel laureate working with the Space Research Institute with the help of the Space Telescope and Johns Hopkins Universities - worked to clarify the value of the Hubble constant, enhancing and improving the process of building the space distance ladder . Together with their team, known as “using supernova H0 to calculate the equation of state” ( Supernova H0 for the Equation of State , SH0ES), they successfully reduced the measurement error of the expansion rate of the Universe to 2.2%.

If in detail, then astronomers traditionally use a scale of distances in astronomy, or a ladder of distances, to measure distances to distant objects of the Universe. It is based on milestones such as variable cepheid stars.- pulsating stars, the distance to which can be calculated by comparing their absolute brightness with the visible [and the absolute brightness to calculate, based on the period of pulsations / approx. trans.]. Such measurements are then compared with the redshift of light coming from distant galaxies to determine how quickly the space between the galaxies expands.

Hence the Hubble constant is derived. Another method is to observe the relic radiation, and to track the expansion of the early Universe — when approximately 378,000 years have passed since the Big Bang — from which modern expansion speed is derived using physics and extrapolation. Together, these methods should provide a schedule for the expansion of the universe from the very beginning to the present day.

However, astronomers have long known that these two measurements do not coincide with each other. In the previous study , when Riesz and the team also conducted research using the Hubble telescope, they obtained a constant value of 73 km / s / Mpc. Meanwhile, the results obtained from the measurements of the Planck Observatory (observing the background radiation from 2009 to 2013) suggest that the Hubble constant should be 67 km / s / Mpc, and certainly not more than 69 - and this discrepancy is 9%.


Relic radiation in pseudo colors

As Riess noted in a recent press release from NASA:

Tension has grown into a real incompatibility of our ideas about the early and late Universe. It became clear that this is no longer a consequence of some terrible error in one of the measurements. It is as if you predicted the growth of a child according to the schedule of growth of people, and then you discovered that, as an adult, he greatly exceeded expectations. We are completely confused.

In this case, Riess and his colleagues used the Hubble telescope to estimate the brightness of remote Cepheids, and Gaia provided parallax data — the apparent change in the object's location depending on the point of view — necessary to determine the distance. Another Gaia contribution was to measure the distance to 50 Cepheids of the Milky Way, which were combined with the Hubble measurements.

This allowed astronomers to more accurately calibrate Cepheids and use those outside the Milky Way as markers. Using measurements obtained from Hubble and new data from Gaia, Riesz and colleagues were able to clarify the measured value of the expansion velocity to 73.5 km / s / Mpc.


The satellite of Gaia of the European Space Agency is currently fulfilling its five-year mission to build a map of the stars of the Milky Way.

Stefano Casertano from the Space Research Institute using the space telescope and a member of the SH0ES team added:

Hubble does an amazingly good job with the role of a general-purpose observatory, but Gaia is a new standard for distance calibration. It is specially created for measuring parallax - it was developed for this. Gaia provides new opportunities for recalibrating all previous measured distances and confirms our previous work. We get the same value for the Hubble constant, replacing all previous distance scale calibrations with just parallax values ​​obtained from Gaia. This is a cross check of two most powerful and accurate observatories.

In the future, Riess and his team hope to continue working with Gaia to reduce the uncertainty associated with Hubble’s constant to 1% by the early 2020s. In the meantime, the discrepancy between the current expansion rate and that obtained from the relic radiation data will continue to amaze astronomers.

As a result, this may be a sign that some other physics is working in the Universe, that dark matter interacts with normal matter differently than scientists suspected, or that dark energy may turn out to be even more exotic than previously thought. Whatever the reason, it is clear that the universe still has surprises for us!