In the 100th year of Einstein's revolutionary discovery of general relativity, astrophysicists at the Indian Institute of Science (IISc) have reported that the celebrated Chandrasekhar limit, is not "unique" and Einstein's theory for understanding astronomical phenomena might not be the "ultimate"1.
They arrived at this provocative conclusion while trying to find an explanation for the recent observation of several supernovae, or stellar explosions, which are either highly over-luminous or highly under-luminous compared to their standard counterparts.
Discovery of these peculiar supernovae has puzzled astronomers but so far defies satisfactory explanation.
The "Chandrasekhar limit" for white dwarfs is a discovery for which Indian-American scientist Subrahmanyan Chandrasekhar shared the physics Nobel in 1983. White dwarfs are formed after a normal star exhausts all of its hydrogen fuel.
In 1930, when he was barely 19, Chandrasekhar showed (based on simple Newtonian gravity and elementary quantum mechanics) that a white dwarf can have a maximum mass of about 1.4 times the mass of our Sun. This upper mass limit came to be known as Chandrasekhar limit. Any white dwarf with less than this mass stays as a white dwarf forever, while a white dwarf whose mass approaches the Chandrasekhar limit is destined to blow up in a violent thermonuclear explosion called Type Ia supernova (SNeIa).
"Observations of white dwarf explosions generally seem to abide by this Chandrasekhar mass-limit," Banibrata Mukhopadhyay, lead author of the paper told Nature India . "However the discovery of several peculiar SNeIa – some of them highly over-luminous (super-SNeIa) compared to their standard counterparts, and some highly under-luminous (sub-SNeIa) – provoked us to rethink the commonly accepted scenario that a dwarf star explodes only when its mass crosses Chandrasekhar limit," he said.
The existence of super- and sub-SNeIa means the masses of exploding stars can be widely different from this limit, Mukhopadhyay said. "For instance, the former possibly calls for the mass of the white dwarf to be significantly higher than the limit set by Chandrasekhar."
This anomaly cannot be satisfactorily explained either by the Newtonian or Einstein's theory of gravity, and no detailed analysis has been performed to describe all the classes of SNeIa by a single theory, Mukhopadhyay said. "Hence, we were motivated to look for an alternative theory."
Although white dwarfs could be explained by Einstein's theory, the validity of this theory has been tested mainly in the weak gravity regimes through laboratory experiments and solar system tests. "On the other hand, the expanding universe and the region close to a black hole and neutron stars are the regimes of strong gravity," Mukhopadhyay said. "Hence the theory of gravitation may require modification in the high density, strong gravity regime of white dwarfs."
"Applying the theory of modified Einstein's gravity, we not only obtain both highly super-Chandrasekhar (2.1−2.8 solar mass) and highly sub-Chandrasekhar (0.5-1.2 solar mass) limiting mass white dwarfs, but also establish them as progenitors of the peculiar, over-luminous and under-luminous SNeIa, respectively," Mukhopadhyay said. In other words, he said, a modification to Einstein's gravity makes the limiting mass of white dwarfs variable and therefore not unique.
"Thus, a single underlying theory, inspired by the need to modify Einstein’s theory of general relativity, unifies the two apparently disjoint sub-classes of SNeIa, which have so far hugely puzzled astronomers," their report said.
The work not only questions the uniqueness of the Chandrasekhar mass-limit for white dwarfs, but it also argues for the possible need for a modified theory of gravity to explain astrophysical observations.
However, some astrophysicists disagree. "The value of Chandrasekhar limit (1.44 solar mass) is obtained by assuming that the white dwarf is non-rotating, non-magnetic and having no significant thermal or radiation pressure," Abhas Mitra, head of theoretical astrophysics group at the Bhabha Atomic Research Centre in Mumbai told Nature India .
"When these conditions are relaxed, one can have a range of limiting masses for the white dwarf without having to replace Einstein’s General Relativity (GR) theory with a cumbersome new theory.” Mitra said many such new gravity theories have failed various tests for solar system motions.
