Editor's
Note: We asked several scientists from various fields what
they thought were the greatest mysteries today, and then we added a few that
were on our minds, too. This article is one of 15 in LiveScience's
"Greatest Mysteries" series running each weekday.
How did the
universe come to be?
It is perhaps
the greatest Great Mystery, and the root of all the others. The rest of
humanity's grand questions—How did life begin? What is consciousness?
What is dark matter, dark energy, gravity?—stem from it.
"All
other mysteries lie downstream of this question," said Ann Druyan, the author
and widow of astronomer Carl Sagan. "It matters to me because I am human
and do not like not knowing."
Even as the
theories attempting to solve this mystery grow increasingly complex, scientists
are haunted by the possibility that some of the most critical links in their
chain of reasoning is wrong.
Fundamental
mysteries
According
to the standard Big
Bang model, the universe was born during a period of inflation that began
about 13.7 billion years ago. Like a rapidly expanding balloon, it swelled from
a size smaller than an electron to nearly its current size within a tiny
fraction of a second.
Initially,
the universe was permeated only by energy. Some of this energy congealed into particles,
which assembled into light atoms like hydrogen and helium. These atoms clumped
first into galaxies, then stars, inside whose fiery furnaces all the other
elements were forged.
This is the
generally agreed-upon picture of our universe's origins as depicted by
scientists. It is a powerful model that explains many of the things scientists
see when they look up in the sky, such as the remarkable smoothness of
space-time on large scales and the even distribution of galaxies on opposite
sides of the universe.
But there
are things about it that make some scientists uneasy. For starters, the idea
that the universe underwent a period of rapid inflation
early in its history cannot be directly tested, and it relies on the existence
of a mysterious form of energy in the universe's beginning that has long disappeared.
"Inflation
is an extremely powerful theory, and yet we still have no idea what caused
inflation—or whether it is even the correct theory, although it works extremely
well," said Eric Agol, an astrophysicist at the University of Washington.
For some
scientists, inflation is a clunky addition to the Big Bang model, a necessary
complexity appended to make it fit with observations. Nor was it the last such
addition.
"We've
also learned there has to be dark matter in the universe, and now dark
energy," said Paul Steinhardt, a theoretical physicist at Princeton University. "So the way the model works today is you say, 'OK, you take some
Big Bang, you take some inflation, you tune that to have the following
properties, then you add a certain amount of dark matter and dark energy.'
These things aren't connected in a coherent theory."
"What's
disturbing is when you have a theory and you make a new observation, you have
to add new components," Steinhardt added. "And they're not connected
... There's no reason to add them, and no particular reason to add them in that
particular amount, except the observations. The question is how much you're
explaining and how much you're engineering a model. And we don't' know
yet."
An
ageless universe
In recent
years, Steinhardt has been working with colleague Neil Turok at Cambridge University on a radical alternative to the standard Big Bang model.
According
to their idea, called the ekpyrotic
universe theory, the universe was born not just once, but multiple times in
endless cycles of fiery death and rebirth. Enormous sheet-like
"branes," representing different parts of our universe, collide about
once every trillion years, triggering Big Bang-like explosions that re-inject
matter and energy into the universe.
The pair
claims that their ekpyrotic, or "cyclic," theory would explain not
only inflation, but other cosmic mysteries as well, including dark matter, dark
energy and why the universe appears to be expanding at an ever-accelerating
clip.
While
controversial, the ekpyrotic theory raises the possibility that the universe is
ageless and self-renewing. It is a prospect perhaps even more awe-inspiring
than a universe with a definite beginning and end, for it would mean that the
stars in the sky, even the oldest ones, are like short-lived fireflies in the
grand scheme of things.
"Does
the universe resemble any of the physical models we make of it? I'd like to
hope that the effort society pours into scientific research is getting us
closer to fundamental truths, and not just a way to make useful tools,"
said Caltech astronomer Richard Massey. "But I'm equally terrified of
finding out that everything I know is wrong, and secretly hope that I
don't."
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