Normal matter
4.9%Dark matter
26.8%Dark
energy
68.3%60 65 70 75 80CMB LOCAL UNIVERSEPlanck
(2018) SH0ES (2018)Carnegie (2018, H band)Carnegie (2018, B band)H0LiCOW (2020)Hubble constant (km/s/Mpc)28 ASTRONOMY • MAY 2020
matter and other forms of energy it con-
tains. When cosmologists infer the value
of the Hubble constant from the cosmic
microwave background, they have to
make assumptions about the amounts
of dark matter, neutrinos, and other
substances that were present.
Perhaps the simplest way to explain
the tension between the different mea-
surements of the Hubble constant would
be to hypothesize that the cosmos con-
tained more energy than expected during
the first hundred thousand years or so
following the Big Bang. This energy
might have taken the form of an exotic
species of light and feebly interacting
particles, or of some kind of dark energy
associated with the vacuum of space
itself that has long since disappeared
from the universe. Or perhaps there is
something else we don’t understand
about this era of cosmic history. We
simply do not yet know how to resolve
this intriguing mystery.
Is a revolution coming?
As I said earlier, it’s possible that the
various puzzles cosmologists face today
are little more than a few trivial threads
that scientists will tie up nicely in the
years ahead with the help of new experi-
ments and observations. But lately, it
seems the more we study the universe,
the less we understand it. Despite
decades of effort, the nature of dark
matter remains unknown, and the prob-
lem of dark energy seems nearly intrac-
table. We do not know how the particles
that make up the atoms in our universe
managed to survive the first moments
of the Big Bang, and we still know
little about cosmic inf lation, how it
played out, or how it came to an end —
assuming that something like inf lation
happened at all.It is from this perspective that I some-
times find myself considering whether
these mysteries might represent some-
thing greater than a few open and unre-
lated questions. Perhaps they are telling
us that the earliest moments of our uni-
verse were far different from what we
long imagined them to be. Perhaps these
problems represent the beginning of a
revolution for the science of cosmology.
Sometimes I wonder whether we
might be on a significant precipice of
scientific history, similar to what we
experienced in 1904. At that time, phys-
ics had never before seemed to be on
such solid footing. For more than two
centuries, the principles of Newtonian
physics had been applied successfully to
problem after problem. And although
physicists expanded their knowledge into
areas such as electricity, magnetism, and
heat, these aspects of the world were
really not so different from those Newton
had described hundreds of years earlier.
To the physicists of 1904, the world
seemed well understood. There was little
reason to expect a revolution.
Similar to the situation cosmologists
confront today, however, the physicists of
1904 had not yet been able to address a
few challenges. The medium through
which they believed light traveled —
the luminiferous ether — should have
induced variations in the speed of light,
and yet light always moves through spaceAstronomers have been trying to determine the expansion rate of the
cosmos for nearly a century. Although measurements of this so-called
Hubble constant have grown more precise over the years, different
methods yield different results. Direct observations of relatively nearby
galaxies give significantly higher values than those deduced from
observations of the cosmic microwave background. ASTRONOMY: ROEN KELLYA type Ia supernova exploded in the grand design spiral M100 in early
February 2006. (The supernova is the brighter of the two stars to the lower
right of the galaxy’s center.) Supernovae of this type, which arise when a
white dwarf accumulates too much mass from a binary companion, all
have the same intrinsic brightness and thus make excellent distance
indicators. ESO/IDA/DANISH 1.5 M/R. GENDLER, J.-E. OVALDSEN, C.C. THÖNE, AND C. FÉRONTENSION
IN THE
COSMOSWHAT IS THE UNIVERSE
MADE OF?The atoms that make up stars, planets, and
people add up to less than 5 percent of the
universe’s constituents. Invisible dark matter
contributes more than five times as much,
while the dark energy that powers the
accelerating cosmos accounts for more
than two-thirds of the total. ASTRONOMY: ROEN KELLY