June 02, 2016
This illustration shows
the three steps astronomers used to measure the
universe's expansion rate to an unprecedented
accuracy, reducing the total uncertainty to 2.4
Astronomers made the
measurements by streamlining and strengthening the
construction of the cosmic distance ladder, which is
used to measure accurate distances to galaxies near
and far from Earth.
Beginning at left,
astronomers use Hubble to measure the distances to a
class of pulsating stars called Cepheid variables,
employing a basic tool of geometry called parallax.
This is the same
technique that surveyors use to measure distances on
calibrate the Cepheids' true brightness, they can
use them as cosmic yardsticks to measure distances
to galaxies much farther away than they can with the
The rate at which
Cepheids pulsate provides an additional fine-tuning
to the true brightness, with slower pulses for
The astronomers compare
the calibrated true brightness values with the
stars' apparent brightness, as seen from Earth, to
determine accurate distances.
Once the Cepheids are
calibrated, astronomers move beyond our Milky Way to
nearby galaxies (shown at center). They look for
galaxies that contain Cepheid stars and another
reliable yardstick, Type Ia supernovae, exploding
stars that flare with the same amount of brightness.
The astronomers use the
Cepheids to measure the true brightness of the
supernovae in each host galaxy. From these
measurements, the astronomers determine the
They then look for
supernovae in galaxies located even farther away
from Earth. Unlike Cepheids, Type Ia supernovae are
brilliant enough to be seen from relatively longer
The astronomers compare
the true and apparent brightness of distant
supernovae to measure out to the distance where the
expansion of the universe can be seen (shown at
They compare those
distance measurements with how the light from the
supernovae is stretched to longer wavelengths by the
expansion of space.
They use these two
values to calculate how fast the universe expands
with time, called the Hubble constant.
ESA, A. Feild (STScI), and A. Riess (STScI/JHU)
Astronomers using NASA's Hubble Space Telescope have discovered that
the universe is expanding 5 percent to 9 percent faster than
"This surprising finding may be an
important clue to understanding those mysterious parts of the
universe that make up 95 percent of everything and don't emit
light, such as
dark energy, dark matter, and
dark radiation," said study
leader and Nobel Laureate Adam Riess of the Space Telescope
Science Institute and The Johns Hopkins University, both in
The results (A
2.4% Determination of the Local Value of the Hubble Constant)
will appear in an upcoming issue of The Astrophysical Journal.
Adam Riess' team made the discovery by refining the
universe's current expansion rate to unprecedented accuracy,
reducing the uncertainty to only 2.4 percent.
The team made the refinements by
developing innovative techniques that improved the precision of
distance measurements to faraway galaxies.
The team looked for galaxies containing both
Cepheid stars and
Type Ia supernovae.
Cepheid stars pulsate at rates
that correspond to their true brightness, which can be
compared with their apparent brightness as seen from Earth
to accurately determine their distance.
Type Ia supernovae, another
commonly used cosmic yardstick, are exploding stars that
flare with the same brightness and are brilliant enough to
be seen from relatively longer distances.
By measuring about 2,400 Cepheid stars
in 19 galaxies and comparing the observed brightness of both types
of stars, they accurately measured their true brightness and
calculated distances to roughly 300 Type Ia supernovae in far-flung
The team compared those distances with the expansion of space as
measured by the stretching of light from receding galaxies. The team
used these two values to calculate how fast the universe expands
with time, or the Hubble constant (or
The improved Hubble constant value is 73.2 kilometers per
second per megaparsec. (A megaparsec equals 3.26 million
The new value means the distance between
cosmic objects will double in another 9.8 billion years.
This refined calibration presents a puzzle, however, because it does
not quite match the expansion rate predicted for the universe from
its trajectory seen shortly after the big bang.
Measurements of the afterglow from the
big bang by NASA's Wilkinson Microwave Anisotropy Probe
and the European Space Agency's
Planck satellite mission yield
predictions for the Hubble constant that are 5 percent and 9 percent
"If we know the initial amounts of
stuff in the universe, such as
dark energy and dark matter,
and we have the physics correct, then you can go from a
measurement at the time shortly after the big bang and use that
understanding to predict how fast the universe should be
expanding today," said Riess.
"However, if this discrepancy holds
up, it appears we may not have the right understanding, and it
changes how big the Hubble constant should be today."
This animation shows
the principle of the cosmic distance ladder used
by Adam Riess and his team to reduce the
uncertainty of the Hubble constant.
NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU)
Comparing the universe's expansion rate with WMAP, Planck, and
Hubble is like building a bridge, Riess explained.
On the distant shore are the cosmic
microwave background observations of the early universe. On the
nearby shore are the measurements made by Riess' team using Hubble.
"You start at two ends, and you
expect to meet in the middle if all of your drawings are right
and your measurements are right," Riess said. "But now the ends
are not quite meeting in the middle and we want to know why."
There are a few possible explanations
for the universe's excessive speed.
One possibility is that dark energy,
already known to be accelerating the universe, may be shoving
galaxies away from each other with even greater - or growing -
Another idea is that the cosmos contained a new subatomic particle
in its early history that traveled close to the speed of light. Such
speedy particles are collectively referred to as "dark radiation"
and include previously known particles like neutrinos.
More energy from additional dark
radiation could be throwing off the best efforts to predict today's
expansion rate from its post-big bang trajectory.
The boost in acceleration could also mean that dark matter possesses
some weird, unexpected characteristics. Dark matter is the backbone
of the universe upon which galaxies built themselves up into the
large-scale structures seen today.
And finally, the speedier universe may be telling astronomers that
Einstein's theory of gravity is incomplete.
"We know so little about the dark
parts of the universe, it's important to measure how they push
and pull on space over cosmic history," said Lucas Macri of
Texas A&M University in College Station, a key collaborator on
The Hubble observations were made with
Hubble's sharp-eyed Wide Field Camera 3 (WFC3), and were
conducted by the Supernova H0 for the Equation of State
(SH0ES) team, which works to refine the accuracy of the Hubble
constant to a precision that allows for a better understanding of
the universe's behavior.
The SH0ES Team is still using Hubble to reduce the uncertainty in
the Hubble constant even more, with a goal to reach an accuracy of 1
Current telescopes such as the European
Gaia satellite, and future telescopes such as the
James Webb Space Telescope (JWST), an infrared observatory, and
the Wide Field Infrared Space Telescope (WFIRST), also could
help astronomers make better measurements of the expansion rate.
Before Hubble was launched in 1990, the estimates of the Hubble
constant varied by a factor of two.
In the late 1990s the Hubble Space
Telescope Key Project on the Extragalactic Distance Scale
refined the value of the Hubble constant to within an error of only
10 percent, accomplishing one of the telescope's key goals.
The SH0ES team has reduced the
uncertainty in the Hubble constant value by 76 percent since
beginning its quest in 2005.
Hubble finds universe may be expanding faster