22 ASTRONOMY • MAY 2020
A
series of powerful
observations has made
it clear that our uni-
verse has expanded for
billions of years,
emerging from the
hot, dense state we call the Big Bang.
Over the past several decades, new types
of precise measurements have allowed
scientists to scrutinize and refine this
account, letting them reconstruct the his-
tory of our universe in ever greater detail.
When we compare the results from dif-
ferent kinds of measurements — the
expansion rate of the universe, the tem-
perature patterns in the light released
when the first atoms formed, the
abundances of various chemical elements,
and the distribution of galaxies and other
large-scale structures — we find stunning
agreement. Each of these lines of evidence
supports the conclusion that our universe
expanded and evolved in just the way that
the Big Bang theory predicts. From this
perspective, our universe appears to be
remarkably comprehensible.
But cosmologists have struggled —
if not outright failed — to understand
essential facets of the universe. We know
almost nothing about dark matter and
dark energy, which together make up
more than 95 percent of the total energy
in existence today. We don’t understand
how the universe’s protons, electrons,and neutrons could have survived the
aftereffects of the Big Bang. In fact,
everything we know about the laws of
physics tells us that these particles should
have been destroyed by antimatter long
ago. And in order to make sense of the
universe as we observe it, cosmologists
have been forced to conclude that space,
during its earliest moments, must have
undergone a brief and spectacular period
of hyperfast expansion — an event
known as cosmic inf lation. Yet we know
next to nothing about this key era of
cosmic history.
It’s possible that these puzzles are little
more than loose ends, each of which will
be resolved as cosmologists continue toWith a diameter 2.5 times
that of the Milky Way and
containing 10 times as
many stars, UGC 2885
ranks among the largest
spiral galaxies in the local
universe. But there’s more
here than meets the eye:
Invisible dark matter
accounts for some 85
percent of UGC 2885’s
mass. Vera Rubin
discovered dark matter
in this galaxy during her
pioneering study of this
mysterious material.
NASA/ESA/B. HOLWERDA (UNIVERSITY
OF LOUISVILLE)