Cosmic Inflation
Initial Expansion
Around 13.8 billion years ago, the universe expanded faster
than the speed of light for a fraction of a second, a period
called cosmic inflation. Scientists aren’t sure what came
before inflation or what powered it. It’s possible that energy
during this period was just part of the fabric of space-time.
Cosmologists think inflation explains many aspects of the
universe we observe today, like its flatness, or lack of
curvature, on the largest scales. Inflation may have also
magnified density differences that naturally occur on space’s
smallest, quantum-level scales, which eventually helped form
the universe’s large-scale structures.
First Particles
Neutrons, protons, and electrons form
When cosmic inflation stopped, the energy driving it
transferred to matter and light – the big bang. One second
after the big bang, the universe consisted of an extremely hot
(18 billion degrees Fahrenheit or 10 billion degrees Celsius)
primordial soup of light and particles.
First Nuclei
Helium and hydrogen form
In the following minutes, an era called nucleosynthesis,
protons and neutrons collided and produced the earliest
elements – hydrogen, helium, and traces of lithium and
beryllium. After five minutes, most of today’s helium had
formed, and the universe had expanded and cooled enough that
further element formation stopped. At this point, though, the
universe was still too hot for the atomic nuclei of these
elements to catch electrons and form complete atoms. The
cosmos was opaque because a vast number of electrons created a
sort of fog that scattered light.
First Light
The first atoms form
Around 380,000 years after the big bang, the universe had
cooled enough that atomic nuclei could capture electrons, a
period astronomers call the epoch of recombination. This had
two major effects on the cosmos. First, with most electrons
now bound into atoms, there were no longer enough free ones to
completely scatter light, and the cosmic fog cleared. The
universe became transparent, and for the first time, light
could freely travel over great distances. Second, the
formation of these first atoms produced its own light. This
glow, still detectable today, is called the cosmic microwave
background. It is the oldest light we can observe in the
universe. After the cosmic microwave background, the universe
again became opaque at shorter wavelengths due to the
absorbing effects of all those hydrogen atoms. For the next
200 million years the universe remained dark. There were no
stars to shine. The cosmos at this point consisted of a sea of
hydrogen atoms, helium, and trace amounts of heavier elements.
First Stars
Gas and dust condense into stars
Gas was not uniformly distributed throughout the universe.
Cooler areas of space were lumpier, with denser clouds of gas.
As these clumps grew more massive, their gravity attracted
additional matter. As they became denser, and more compact,
the centers of these clumps became hotter – hot enough
eventually that nuclear fusion occurred in their centers.
These were the first stars. They were 30 to 300 times more
massive than our Sun and millions of times brighter.
Galaxies & Dark Matter
Galaxies form in dark matter cradles
Over several hundred million years, the first stars collected
into the first galaxies. At first, starlight couldn’t travel
far because it was scattered by the relatively dense gas
surrounding the first stars. Gradually, the ultraviolet light
emitted by these stars broke down, or ionized, hydrogen atoms
in the gas into their constituent electrons and protons. As
this reionization progressed, starlight traveled farther,
breaking up more and more hydrogen atoms. By the time the
universe was 1 billion years old, stars and galaxies had
transformed nearly all this gas, making the universe
transparent to light as we see it today.
Dark Energy
Expansion accelerates
For many years, scientists thought the universe’s current
expansion was slowing down. But in fact, cosmic expansion is
speeding up. In 1998, astronomers found that certain
supernovae, bright stellar explosions, were fainter than
expected. They concluded this could only happen if the
supernovae had moved farther away, at a faster rate than
predicted. Scientists suspect a mysterious substance they call
dark energy is accelerating expansion. Future research may
yield new surprises, but cosmologists suggest it’s likely the
universe will continue to expand forever.
Today
Humans observe the universe