Two of the most important mysteries of both string theory
and cosmology area unit the presence of unseen substance and of repulsive
gravity within the form of dark energy.
In the Nineteen Thirties, Swiss astronomer Fritz Zwicky 1st
determined that some galaxies were spinning so fast that the stars in them
ought to fly off from each other. unfortunately, Zwicky had temperament clashes
with several within the natural philosophy community, so his views weren’t
taken terribly seriously.
In 1962, astronomer Vera Rubin created a similar discoveries
and had nearly a similar outcome. although Rubin didn’t have a similar issues
of temperament that Zwicky did, several forgotten her work as a result of she
was a lady.
Rubin maintained her focus on the matter and, by 1978, had
studied 11 spiral galaxies, all of which (including our own whitish Way) were
spinning so fast that the laws of physics said they should fly apart. together
with work from others, this was enough to persuade the natural philosophy
community that one thing strange was happening.
Whatever is holding these galaxies along, observations
currently indicate that there must be far more of it than there's the visible
matter that makes up the baryonic matter that we’re used to — the matter that
comprises you, your laptop, this planet, and therefore the stars.
Physicists have created several suggestions about what might
structure this substance, however to this point nobody is aware of for sure.
Dark energy: Pushing the universe apart
Einstein’s cosmological constant allowed for a consistent
repulsive energy throughout the universe. Since Hubble discovered the expansion
of the universe, most scientists have believed that the cosmological constant
was zero (or presumably slightly negative). Recent findings have indicated that
the expansion rate of the universe is really increasing, which means that the
cosmological constant encompasses a positive value. This repulsive gravity — or
dark energy — is really pushing the universe apart. this is often one major
feature of the universe that string theory may be ready to explain.
In 1998, 2 groups of astronomers announced a similar
results: Studies of distant supernovas (exploding stars) showed that stars
looked dimmer than expected. the sole way to account for this was if the stars
were somehow farther away than expected, however the physicists had already
accounted for the expansion of the universe. the reason eventually found was
startling: the rate of expansion of the universe was accelerating.
To explain this, physicists realised that there had to be
some variety of repulsive gravity that worked on large scales (see the figure,
below). On tiny scales, normal gravity rules, however on larger scales the
repulsive gravity force of dark energy seemed to take over. (This doesn’t
contradict the thought that the universe is flat — however it makes the fact
that it is flat, whereas still expanding, a really uncommon and unexpected set
of circumstances, which required terribly slim parameters on the early
conditions of our universe.)
Repulsive gravity is theorized by inflation theory, but that’s a speedy hyper-expansion within the early phases of the universe. Today’s enlargement owing to dark energy could also be remnants of the repulsive gravity from inflation, or it may be an entirely distinct phenomenon.
The finding of dark energy (or a positive cosmological constant, which it is roughly similar to) creates major theoretical hurdles, particularly considering however weak dark energy is. For years, quantum field theory foreseen an enormous cosmological constant, but most physicists assumed that some property (such as supersymmetry, which will cut back the cosmological constant value) canceled it intent on zero. Instead, the value is non-zero, but differs from theoretical predictions by nearly 120 decimal places!
In fact, results from the WMAP show that the overwhelming majority of fabric in our gift universe — regarding 73 percent — is created up of dark energy (remember from Einstein's theory of relativity that matter and energy area unit totally different styles of constant thing: E = mc2, after all). The five-year WMAP data, free in 2008 and shown within the figure, below, also allows you to compare the composition of {the gift|this|the current} universe with the material present within the universe thirteen.7 billion years past. The dark energy was a vanishingly tiny slice of the pie thirteen.7 billion years past, but these days it eclipses matter and drives the universe’s enlargement.
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