- Astronomy: a cosmic bat
Astronomer Rafael Bachiller discovers in this series the most spectacular phenomena of the Cosmos. Topics of throbbing research, astronomical adventures and scientific news about the Universe analyzed in depth.
A constant that is not constant
During the last hundred years, since the very discovery of the big bang, the expansion of the universe has been the subject of heated debates that have gone through ups and downs, but are taking a new verve today. Astronomers do not agree on the rate of expansion of the universe. These debates have focused on the value of the Hubble constant, the parameter that measures the rate of expansion that, roughly, is around 70 kilometers per second and per megaparsec (km / s / Mpc).
At least, you can say that these are rare units. A megaparsec is a unit of distance that equals 3.26 million light years, a convenient magnitude when we consider the large-scale structure of the universe. But what physical significance does the Hubble constant have? certainly all this requires an explanation but, in order not to lose the thread, we offer that explanation (which only contains one formula, and easy) at the end of this article.
Only two decades ago it was discovered that the expansion of the universe is accelerated, and this, together with the fact that the universe is diluting as it expands, translates into the fact that Hubble's 'constant' is slightly changing as it progresses time. So, to express ourselves properly, we should not talk about Hubble's constant, but about Hubble's 'parameter' (H). Here, to simplify, we will focus on the current value of H, that is, at the present time, this value of H in the current (local) universe is designated by Ho.
A century of controversy
Shortly after the discovery of the big bang, Ho's first estimates were so crude that the age that resulted for the universe was shorter than the age of the Earth. The great astronomer Henrietta Leavitt developed a method that used variable stars (the so-called cepheids) as beacons to indicate the distances of the galaxies. But even so, with this powerful method, just 30 years ago, Ho's measurements came from two families of astronomers who seemed irreconcilable. A series of astronomers measured values around 50 km / s / Mpc, while another school found values on the order of 100 km / s / Mpc.
Finally, in 2001, another American astronomer, Wendy Freeman (Univ. Chicago), observing a large number of cepheids with the Hubble space telescope determined a much more reliable value: 72 km / s / Mpc, with an uncertainty of 10%. In recent years, these measures have been refined more and more, cosmology has become a science of precision. In addition to cepheids, other distance indicators (such as type Ia supernovae) have been included. And thanks to all this, Ho = 73.5 km / s / Mpc has been determined, with only 2% uncertainty.
Meanwhile, the discovery of the accelerated expansion of the universe led to a cosmological model, called CDM, in which dark energy (designated by) is responsible for the acceleration of the universe and represents 68.4% of the total composition of the universe . 26.5% would consist of cold dark matter (CDM) and 4.9% ordinary matter. To this we must add the mass of neutrinos (poorly known) and other effects to reach 100% total. Using this model and the fantastic data obtained by the ESA Planck satellite, the current Ho value can be extrapolated from the primitive universe, which turns out to be 67.4 km / s / Mpc, with 1% uncertainty. And other data obtained by the cosmological observation project 'Dark Energy Survey (DES) get exactly that same value, although with an uncertainty of 2%.
Tense debate
It should be stressed that, although all measurements refer to the current Ho value, the first (with cepheids and supernovae) are made using data from the current (local) universe, while the second measurements (with Planck and DES) use data from the universe primitive and extrapolate the value of H to the current universe. Well, the differences between both sets of measures are very statistically significant: the measurements made in the local universe show a value of Ho that is 9% higher than the extrapolation of the data from the primitive universe.
And again it seems that there is no possible reconciliation. All astronomers argue about the high quality of their measurements, the goodness of their method, and say they have repeated a thousand times the processes of data reduction. The debate was staged at an important congress that brought together 130 cosmologists a few months ago in Berlin, including some of the most prestigious in the world and the occasional Nobel Prize. These long discussions continue in the scientific literature and in all possible occasions, and monopolize the present in cosmology. At the moment there is no possible agreement. This debate, which, as we see, has reached large dimensions, has been renamed the Hubble Tension.
One part in ten billion
Some theoretical cosmologists already squander imagination trying to find a possible way out. Some argue that dark matter could be destroyed as the universe evolves or, at least, lose some of its ability to slow down the expansion. Others look for reason in variations in the density of dark matter. All these ideas do not go beyond speculation at the moment, but there is a growing consensus that some ingredient of what has come to be called "new physics" will be necessary to resolve the controversy. It may be necessary to slightly modify some of the fundamental laws of physics, admitting for example small variations in the law of gravity.
In my opinion, I consider it essential to continue with the observations. For example, Wendy Freeman has just published a new study, based on observations of giant red stars, which finds an intermediate value between the previous two (69.8 km / s / Mpc) with an uncertainty of 2%. A value that adds more fuel to the fire of the debate.
We must achieve more precision in the observations and eliminate all sources of error that may be found in the interpretive processes. For this, it would be ideal to be able to measure the expansion directly. That is, measure the distance to a distant galaxy, repeat the measurement after a while and thus obtain the expansion directly, without any interpretation exercise, or extrapolations from the primitive universe. The problem is that, as we noted at the end of this article, the expansion of the universe in a year is somewhat less than one part in ten billion. Separating the measures for ten years we would have to appreciate a part in a billion. This, which is completely impossible today, may be feasible within ten or twenty years with the large telescopes and radio telescopes (such as the ELT or SKA) that are now under construction.
Also interesting: the Hubble-Lemaître Law
The expansion of the universe is expressed by the Hubble-Lemaître Law, a very simple equation that linearly relates the speed (V) at which any two galaxies are separated with the distance (d) that mediates between them: V = H xd, where H is the Hubble constant. We see that expansion cannot be described with a single speed, nature is a bit more complex. Actually when we talk about the expansion of the universe we are referring to the expansion of space that was created in the big bang. Similar to what happens with the expansion of a metal, it is appropriate to describe the expansion of space with a coefficient, the Hubble constant, which to some extent plays the role of that coefficient of expansion.
Assume that H = 70 km / s / Mpc. For example, two galaxies separated by 100 megaparsecs are separating at a speed V = 7000 km / s. Two other galaxies that are 101 megaparsecs away will separate at 7070 km / s. And for each additional megaparsec, the speed will be increased by 70 km / s. Therefore, in one million years, two galaxies initially separated by 100 megaparsecs will have separated 7,000 additional parsecs (about 23,000 light years). In one year, each length L of the space expands by a percentage slightly less than one part in ten billion.
Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.
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