Cosmic Microwave Background RadiationNASA-WMAP |

**, which measures the speed of expansion of space in the universe, has very curious properties. For instance, although we call it**

*The Hubble constant***, it turns out that it is not a constant, as it varies over time. That is why its current value is represented by the symbol H**

*constant*_{0}, but since its value was different at other times, it can be represented by other symbols, such as H

_{CMBR}, which refers to its value at the time when the

**originated, about 13.7 billion years ago.**

*cosmic microwave background radiation*
One of the hot issues of research is the current value of the Hubble
constant. The way to calculate it seems quite simple: it consists in dividing

**in our vicinity by***the speed of expansion of space***. To do this, the classic way to do it is to measure the distance of a relatively close galaxy (a few hundred light-years), and measure the speed with which it moves away from us. The second value is divided by the first, and we have the approximate value of H***the length of that space*_{0}.
But the analysis of the cosmic background radiation thanks to the COBE,
WMAP and Planck satellites provided another independent way to calculate the
Hubble constant, which consists in

**, adjusting it to the data provided by the satellites about the acoustic oscillations in said radiation, and***proposing a standard cosmological model***. In this case its value is not measured directly, but the adjusted standard model applies Einstein’s cosmological equations to calculate it.***deduce from there the value of H*_{0}Edwin Powell Hubble |

The problem is that

**. When variable Cepheid stars or supernovae are used to estimate the distance of a galaxy, and the redshift of the same galaxy to estimate the speed with which it moves away from us (i.e. the speed of expansion of space), we get a value around 74 with an accuracy of plus or minus 1.4 km/sec/Megaparsec. On the other hand, when it is calculated using the standard cosmological model, the value that comes out is 67.4 plus or minus 0.5 km/sec/Mpc.***the two results obtained do not tally*
At first glance it may seem a small difference, but it is almost 10%, while
both independent results claim to achieve much greater accuracy. The situation
for cosmological physics is unsustainable. To explain the discrepancy, two
alternatives have been proposed:

. We must find new methods to perform them, until both calculations fit.*The distance measurements of nearby galaxies are wrong*(called ΛCDM model)*The standard cosmological model*. We must give it up and design another model.*is wrong*

Several new methods have lately been proposed to calculate the distance
of nearby galaxies with greater precision. Among them we can mention the
following:

- Instead of variable cepheids or supernovae,
have been used to estimate the distance. The result was 69.8, halfway between the two conflicting results.*red giants* can also be used, when a galaxy interposes between us and a distant quasar, whose light curves as it passes near the galaxy, giving rise to a multiple image, from which the distance of the galaxy can be deduced. This method has resulted in H*Gravitational lenses*_{0}= 73.3, very close to that obtained by traditional methods.- Another team has calculated the distance from
, gas clouds revolving around a black hole and emitting light of a certain wavelength. The result obtained was again equal to 74.*megamasers* - Another method based on
resulted in the value 76.5.*image pixelation* - A value of 73.6 was obtained from the use of certain special stars
(
) instead of cepheids.*Miras* - Finally, a new
method based on the analysis of recently discovered
has given the intermediate result of 70, although so far only a single experiment has been possible, so the accuracy is much lower than with the others methods (plus or minus 10).*gravitational waves*

In conclusion: most of the methods based on the direct measurement of
the current value of the Hubble constant are grouped around the value 74. The
standard cosmological model gives a much lower value: 67.4. Two of the new
methods give intermediate values, close to 70, although one of them (gravitational
waves) cannot yet be considered accurate.

This discrepancy, which can be seen graphically in a figure in this
Science News article that shows simultaneously the different results and their accuracy, has
led many astronomers to consider whether the time has come to review the
standard cosmological model, which also presents other problems, like those I
mentioned in another
post in this blog.

**The same post in Spanish**

**Manuel Alfonseca**

## No comments:

## Post a Comment