The universe is expanding, and its expansion is accelerating. We have known that it expands for just under a century, Edwin Hubble discovered it, and the expansion led to the Big Bang. But only since the late 1990s have we discovered that not only is it expanding, but its expansion is accelerating rather than decreasing.
What is this acceleration due to? It is always called dark energy, which we never knew what it is and it makes up more than 70% of the Universe. Now a study has been released by a group of New Zealand scientists who would have solved the problem, explaining it simply with gravity. I wanted to comment, but I thought we need an astrophysicist with balls here. Or rather, one of our greatest astrophysicists, Patrizia Caraveowho together with her husband, the famous (and unfortunately dead) Nanni Bignamiamong other things he discovered a neutron star called Geminga, because they had been observing it for so many years that his Milanese colleagues said, ghe minga, it’s not there. But it was definitely there.
Among other things, Nanni was a friend of mine like Patrizia, as well as of their daughter, Giulia Bignami (with whom I also wrote my latest novel), a brilliant chemist who works as a researcher in Edinburgh and is also an exceptional writer. In short, a family of geniuses.
The only problem for Patrizia is me, who calls her every other week to always ask her the same question: but is there a limit in the universe? And what can you see from the last planet of the last galaxy? Nothingness? Answer: too complicated to explain. In any case, on this question of the New Zealand study which brings out a solution that thousands of physicists have been working on for a century, I turned to you, and I will avoid asking you the question about the last planet in the last galaxy otherwise you’ll send me to hell, a country outside the Milky Way.
So Patrizia, this study from the University of Canterbury in Christchurch, New Zealand has come out, published in the journal Monthly Notices of the Royal AstronomicalSociety Letters, which would solve one of the biggest astronomical puzzles of the last hundred years, do you think it makes sense?
It is a study outside the classical schemes that interpret the expansion of the universe as a dilation of space that occurs uniformly in all directions. In this case the researchers argue that the Universe does not expand uniformly but rather in “zones” which they tried to identify by using supenovae as standard candles, i.e. a classic method for measuring the distance of galaxies.
The new evidence supports the “timescape” model of cosmic expansion, which has no need for dark energy because the deviation from Hubble’s expansion law is not the result of an accelerating Universe, but rather a consequence of the way we calibrate the time and distance. What they argue is that time flows differently inside galaxies, where there is mass, than in the cosmic vacuum.
They are convinced that the data at their disposal are in agreement with this hypothesis, but it must be said that they are the only ones to interpret the data in this way. Others, using the same data, have not reached these conclusions. However, the zone expansion theory could be put to the test with new data from the European orbital telescope EUCLID. It will take several years, but in the end we will understand better whether the Universe is a homogeneous elastic fabric or whether it has more elastic areas that stretch more.
Would it be compatible with relativity or does it present problems?
The study is observational. He says that the expansion of the Universe is more complex than what is assumed based on the Friedmann equation. However, one of the authors adds: “A simple expansion law consistent with Einstein’s general relativity does not have to obey the Friedmann equation.” In other words, let’s not bother Einstein, at least for the moment.
I seem to have understood that there are hypotheses of this kind about dark matter too, that is, that on very large objects, such as galaxies, gravity could act differently there too, eliminating dark matter.
To eliminate dark matter, which is weighed thanks to its gravitational action, the law of universal gravitation must be modified. There is a family of theories called MOND (for MOdified Newtonian Dynamics) that proposes this, but we are talking about theories that, for the moment, have no experimental confirmation.
Do you believe that a solution will come from particle accelerators, or that there are limits that cannot be reached? I mean: we have had countless findings on relativity, but in these cases what experimental evidence could we have?
Numerous attempts are underway to reveal the particles that make up dark matter. Unfortunately we are still at sea. But that doesn’t mean we should stop looking. Technologies are continually improving and the possible mass of these particles is better defined, but, at the moment, we can only say that the phantom particles cannot be heavier than a certain amount or lighter than a certain amount.
Indirect evidence is also sought in the astronomical field using gamma rays. If the dark matter particles decayed producing gamma rays, these would have a well-defined energy and in the data we should clearly see a concentration of photons at a certain energy, in other words there should be a line in the energy distribution of our photons. We looked for it to death without success, but it must be added that, not knowing the mass of the dark particles, we don’t even know the energy where to look for the line. It may be beyond the range covered by our tool. But the search continues. Let’s not forget that gravitational waves were revealed after 50 years of effort and technology development
Last question: reconciling gravity with quantum physics. Here too do you think we will arrive at a theory of everything? And if so, will it come from a new Einstein or from a collective research effort?
The theory of everything is a dream that engages the best theoretical physicists in the world. I believe it takes intuition but remember that sometimes intuition isn’t enough. Think of Fermat’s theorem, scribbled in 1637 in the margin of a book and then proved in 1993 by Andrew Wiles after it had been the obsession of all the great mathematicians, each of whom had made a small contribution.
I’m painfully surprised you didn’t ask me if AI will solve it. I would have told you that I don’t believe that at all.
Very last question: but the last planet of the last galaxy…
Go to hell!