In 1970 the magazine Physics Today published an article by the great Allan Sandage, Don Cosmology, the successor of Edwin Hubble -the discoverer of the expansion of the universe in 1929- at the Mount Wilson observatory. Its title concisely and precisely defined what cosmology was: “ The search for two numbers ”. Those numbers are the Hubble constant, H 0 , and the deceleration parameter, q 0 . Determining its value is not an easy undertaking. Cosmologists have been trying for more than 90 years.
In central Colorado, where snow swirls down the Sawatch Range in the Rockies, a handful of physicists and cosmologists gather each year in the resort town of Aspen. There snow creeps across the flat roofs of a series of small cedar buildings that house the Physics Center as scientists sit at desks in ski gear discussing the origin and evolution of the universe . Curiously, for years one of the mainstays of the Aspen community of cosmologists was Maurice, the short and cheerful French chef with a Swiss education who took care of feeding them and started all his recipes with 120 grams of butter; the universe, meanwhile, began with a big bang. Now, while all cosmologists knew when to go to the dining room, none could say with certainty when the universe was born.
The age of the universe can be calculated in the same way that we can know when a stone was dropped from the hand if we know its position and speed. For the universe it suffices to estimate its rate of expansion , specified by the Hubble constant . Although it is expressed in some curious units (kilometers per second and per megaparsec), its inverse is the age of the universe. And here is the complication. For half a century, cosmologists have been divided into two camps : those who think that the universe is expanding relatively quickly, with a value of 100 for the constant, and those who favor a slow expansion, with a value of 50.
One of the key projects of the Hubble Space Telescope was to settle the debate once and for all by measuring the distance of 31 distant spiral galaxies. The final conclusion recalls the sentence of old Solomon: the Hubble constant is equal to 74. The universe is therefore 13 billion years old. But the controversy did not end there. The two cosmological schools continue with their swords raised. Each uses a type of distance measurement to confirm its own predictions. Freedman uses all his artillery and a technique with an esoteric name, surface brightness fluctuation (SBF) ―which measures the “lumping” of stars in active galaxies and in the central part of spirals: the farther away they are, the less clumpy they look present―to defend a young universe. Sandage uses his “standard bombs,” Type Ia supernovae — white dwarfs that explode by stealing matter from their companion star — to defend an old universe. Is it possible that both methods are flawed? No. Last year a group of astronomers showed that if the FBS said that galaxy A was twice as far away as galaxy B, supernovae Ia confirmed it.
Why not use both methods in harmony to solve the problem? Maybe, but it’s not that easy. Some astronomers think that we are inside a Hubble bubble, a region that is expanding faster than the universe as a whole. Therefore, all observations inside the bubble are flawed. And since the FBS method cannot be applied to distances as far away as that of supernovae, the value of the Hubble constant continues to hover in limbo between 74 and 58. Of course, most astronomers believe that the real value is more closer to the first than to the second.
If the measurement of the speed of the expansion of the universe gives such headaches, doing the same with its changes is an impossible task. The universe is expanding, but is it speeding up or slowing down? Solving this mystery requires measuring the second number, the deceleration parameter q 0 .
A few years ago, whoever wanted to do it had to weigh the matter that the entire universe contains. If it were only to make a mere count of stars, nebulae and galaxies we would have already obtained it. Now, Einstein said that “nature is subtle, but not malicious” , but in this case, it must be recognized that he has gone too far. By observing the rotation of spiral galaxies and the internal motions of galaxy clusters, astronomers have concluded that much of the universe is in the form of dark matter, which cannot be seen. The problem is so serious that it is not even known what this mysterious matter is made of. However, the discovery of Type Ia supernovae has given reason for hope. If they are observed in very distant galaxies, it is possible to know if the universe was expanding faster than it is now. Two international teams are dedicated to this: the High-Z Supernova Search Team (High-Z) and the Supernova Cosmology Project (SCP). What is worrying is their scarcity: this type of supernovae only appears in a galaxy once every 100 years.
Determining the value of the deceleration parameter is also a test of nine for the commonly accepted cosmological model. It does not say anything about what value the Hubble constant should take, but it does say the value of the deceleration parameter: 0.5. Now, the results of High-Z and SCP suggest that their value is actually lower and, therefore, the universe is not slowing down, but rather accelerating.
Cosmologists have an important problem to solve. Standard cosmological theory predicts that the matter density of the universe is just the critical value that separates a universe collapsing under its own gravity from one that expands indefinitely. But observations of supernovae imply negative matter density, which is ridiculous. In the face of such a disaster, cosmologists are not daunted. In fact, they are very flexible characters and they have decided to introduce a surprising entity that is as impossible to believe as the ether that filled the space of the nineteenth century: the energy of the vacuum . This leads us directly to the physics of the very small, to quantum mechanics. According to the famous uncertainty principle, the vacuum ―understood as the absence of matter and energy― does not exist. It is actually a hotbed of particles that appear and disappear in less time than a breath lasts. The thing is, this “vacuum energy” has a visible effect on the universe, giving it an extra push for expansion. Einstein was the first to introduce this “push”, although for very different reasons: his equations resulted in an expanding universe and then the universe was thought to be static. To stop the expansion, he introduced a cosmic repulsion factor that he called the cosmological constant. When Hubble discovered the expansion, Einstein recognized that entering the cosmological constant had been the biggest mistake of his life. Died in 1929, she has been resurrected in the guise of void energy and is apparently going to be with us for a long time.
