V.Mukhanov and A.Starobinsky were awarded the 2013 Gruber Cosmology
Prize (#) . The following Alexei Starobinsky Laureate Profile (#) provides a brilliant sketch of modern cosmology history.
“In 1979, the universe was in
trouble – at least from a cosmologist’s point of view. Compelling evidence
for the Big Bang theory – an interpretation of the universe as expanding over
time – dated only to the mid-1960s. But already theorists found
themselves confronting a problem that threatened to undermine that
theory: Why is the universe so uniform, or homogeneous, on scales much
greater than the size of its largest structures – the web of superclusters of
galaxies that span hundreds of millions of light-years (a light year being the
distance light travels in a year, or about 6 trillion miles)?
According to the Big Bang
theory, galaxies on the whole are being carried away from one another on the
expansion of space itself, so that no matter where you are in the universe, the
rest of the universe seems to be receding from you. Yet if you look at
the most distant part of the universe in one direction, and the most distant
part of the universe in the opposite direction, they will be remarkably
similar. They’re billions of light years apart, double the distance that
light or any other kind of information could have traveled since the Big Bang,
so how could they “know” to be alike?
Alexei Starobinsky, then a
senior researcher at the Landau Institute for Theoretical Physics in Moscow
He had been trying, instead,
to figure out how the origin of a Big Bang universe might have worked, a task
that took him down the rabbit hole of quantum gravity – the attempt to combine
quantum mechanics and the general theory of relativity. In 1979, he
discovered that the universe could have gone through an extraordinarily rapid
exponential expansion in the first moments of its existence. In the same year
he calculated the generation of gravitational waves during this exponential
expansion.
Shortly after it, the
American physicist Alan Guth proposed a brilliant idea that the stage of the
exponential expansion of the early universe, which he called “inflation,” could
explain the incredible uniformity of our universe and resolve many other
outstanding problems of the Big Bang cosmology. This clarified a potential
significance of the regime of the exponential expansion. However, Guth
immediately recognized that his proposal had a flaw: the world described by his
scenario would become either empty or very non-uniform at the end of inflation.
This problem was solved by Andrei Linde, who introduced several major
modifications of inflationary theory, such as “new inflation” (later also
developed by Albrecht and Steinhardt), “chaotic inflation”, and “eternal
chaotic inflation.” A new cosmological paradigm was born.
Starobinsky’s work inspired
two of his fellow theoreticians in Moscow
“There would always remain
small wiggles, or small inhomogeneities, in the distribution of the matter,”
Mukhanov explains. “But normally these kinds of inhomogeneities are
extremely small.” What would have happened, Mukhanov and Chibisov
wondered, to the inhomogeneities that were present during the exponential
expansion? In 1981, Mukhanov and Chibisov concluded that the
exponentially rapid expansion would stretch tiny quantum fluctuations to an
enormously large size. After that, these fluctuations would grow in amplitude
and become the seeds for the galaxy formation.
“We were thinking we could
take these small inhomogeneities and amplify them in the expanding universe,”
Mukhanov says. He and Chibisov concluded that in a certain sense these
primordial wiggles would be the universe today: the
things that make the universe inhomogeneous on smaller scales; the structures
that make the universe more than empty space.
In 1982, several scientists,
including Starobinsky, outlined a theory of quantum fluctuations generated in
new inflation. This theory was very similar to the theory developed by Mukhanov
and Chibisov in the context of the Starobinsky model. Investigation of
inflationary fluctuations culminated in 1985 in the work by Mukhanov, who developed a
rigorous theory of these fluctuations applicable to a broad class of
inflationary models, including new and chaotic inflation.
Later, cosmologists
calculated how those inhomogeneities would appear in the cosmic microwave
background (CMB), the relic radiation dating to the moment when the universe
was 380,000 years old. At that time, hydrogen atoms and photons (packets of
light) decoupled, leaving a kind of “flashbulb” image that pervades the
universe to this day. Since then, numerous observations of the CMB have
found an exquisite match with Mukhanov and Chibisov’s theoretical predictions,
most recently in the release of data from the European Space Agency’s Planck
observatory.”
