It was the end of a long day of conference talks. Many of the delegates were dozing in their seats. But when Philip Mannheim, the last speaker of the day, stood up and spoke of having dispensed with both dark energy and dark matter, they all sat up. In doing so, Mannheim had taken on both Newton and Einstein at once.
Astrophysicists have noticed that the expansion of the universe is accelerating, which they attribute to an inherent energy of space-time called dark energy. Earlier, Einstein had speculated about a cosmological constant (CC) to denote the energy of space-time, and this is still the favoured explanation for the nature of dark energy. There is one big problem 鈥� theories of particle physics predict that the CC should be more than 10120 times larger than observed 鈥� a value so great that it would blow the universe apart before stars or galaxies could form.
鈥淢aybe it鈥檚 time to give up making the CC small,鈥� says Mannheim, a particle physicist at the University of Connecticut in Storrs, who spoke recently at a conference at Imperial College London. 鈥淛ust accept that it鈥檚 huge, and work from there.鈥�
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鈥淢aybe it鈥檚 time to give up making the cosmological constant small, just accept that it鈥檚 huge and work from there鈥�
Mannheim points out that astrophysicists don鈥檛 measure the value of CC directly. Instead, they deduce it from a quantity that is proportional to CC multiplied by Newton鈥檚 gravitational constant (G), which is thought to be a universal constant of nature. Mannheim says that if G had a smaller value on cosmological scales than measured in lab experiments, it would counteract the effect of a very large CC, thus explaining the observations.
In his model, there are two gravitational constants, bringing two different forces into play. The first is t he familiar GNewton which appears in Einstein鈥檚 equations of general relativity. GNewton dominates on small scales and explains the motion of bodies within the solar system. Mannheim鈥檚 second constant, GCosmo 鈥� deduced from particle physics 鈥� takes over on large cosmological scales. It is has a repulsive, anti-gravity effect, accelerating the expansion of the universe 鈥� just as dark energy is thought to do.
Mannheim鈥檚 fix for the CC problem seems simple, but it comes at a high cost. To introduce two different values of G, he has to throw out Einstein鈥檚 equations of general relativity, replacing them with alternative equations based on particle physics.
Mannheim also claims that his model dispenses with dark matter, which was first proposed when astronomers noticed that stars on the outskirts of galaxies were moving much faster than can be explained by the gravity of visible matter. In his model, by adding the two gravitational forces the observations can be explained without resorting to dark matter, says Mannheim.
Subir Sarkar, an astrophysicist at the University of Oxford, is impressed. Although alternate gravity theories to explain dark matter abound: 鈥淢annheim stands out because he starts from scratch with a mathematically rigorous theory, and the required observations fall out of that,鈥� says Sarkar.
Keith Horne of the University of St Andrew鈥檚, UK, also admires the approach because it uses fundamental principles taken from particle physics. 鈥淚t鈥檚 very elegant and it would be wonderful if it were true, because it could unify particle physics and gravity in a very tidy manner,鈥� he says.
However, cosmologist Tom Shanks of Durham University, UK, points out that the standard model of cosmology explains some things very well, for instance, measurements of the cosmic microwave background (CMB) and the motion of binary pulsars. 鈥淚鈥檓 wondering if Mannheim鈥檚 gravity is compatible with these recent, highly precise observations,鈥� says Shanks.
Mannheim is examining the case of the binary pulsar, and plans to investigate the CMB next. 鈥淚 have a few challenges to address, of course,鈥� he says. 鈥淏ut compare that with the standard cosmological model, which hundreds of cosmologists have worked on, and yet is wrong by 120 orders of magnitude. At least my model鈥檚 doing a whole lot better than that.鈥�
