Using two of the world’s most powerful space telescopes — NASA’s Hubble
and ESA’s Gaia — astronomers have made the most precise measurements to
date of the universe’s expansion rate. This is calculated by gauging the
distances between nearby galaxies using special types of stars called
Cepheid variables as cosmic yardsticks. By comparing their intrinsic
brightness as measured by Hubble, with their apparent brightness as seen
from Earth, scientists can calculate their distances. Gaia further
refines this yardstick by geometrically measuring the distances to
Cepheid variables within our Milky Way galaxy. This allowed astronomers
to more precisely calibrate the distances to Cepheids that are seen in
outside galaxies.
Science: NASA, ESA, and A. Riess (STScI/JHU)
Using the power and synergy of two space telescopes, astronomers have made the most precise measurement to date of the universe’s expansion rate.
The results further fuel the mismatch between measurements for the
expansion rate of the nearby universe, and those of the distant,
primeval universe — before stars and galaxies even existed.
This so-called “tension” implies that there could be new physics
underlying the foundations of the universe. Possibilities include the
interaction strength of dark matter, dark energy being even more exotic
than previously thought, or an unknown new particle in the tapestry of
space.
Combining observations from NASA’s Hubble Space Telescope and the
European Space Agency’s (ESA) Gaia space observatory, astronomers
further refined the previous value for the Hubble constant, the rate at
which the universe is expanding from the big bang 13.8 billion years
ago.
But as the measurements have become more precise, the team’s
determination of the Hubble constant has become more and more at odds
with the measurements from another space observatory, ESA’s Planck
mission, which is coming up with a different predicted value for the
Hubble constant.
Planck mapped the primeval universe as it appeared only 360,000 years
after the big bang. The entire sky is imprinted with the signature of
the big bang encoded in microwaves. Planck measured the sizes of the
ripples in this Cosmic Microwave Background (CMB) that were produced by
slight irregularities in the big bang fireball. The fine details of
these ripples encode how much dark matter and normal matter there is,
the trajectory of the universe at that time, and other cosmological
parameters.
These measurements, still being assessed, allow scientists to predict how the early universe would likely have evolved into the expansion rate we can measure today. However, those predictions don’t seem to match the new measurements of our nearby contemporary universe.
These measurements, still being assessed, allow scientists to predict how the early universe would likely have evolved into the expansion rate we can measure today. However, those predictions don’t seem to match the new measurements of our nearby contemporary universe.
“With the addition of this new Gaia and Hubble Space Telescope data,
we now have a serious tension with the Cosmic Microwave Background
data,” said Planck team member and lead analyst George Efstathiou of the
Kavli Institute for Cosmology in Cambridge, England, who was not
involved with the new work.
“The tension seems to have grown into a full-blown incompatibility
between our views of the early and late time universe,” said team leader
and Nobel Laureate Adam Riess of the Space Telescope Science Institute
and the Johns Hopkins University in Baltimore, Maryland. “At this point,
clearly it’s not simply some gross error in any one measurement. It’s
as though you predicted how tall a child would become from a growth
chart and then found the adult he or she became greatly exceeded the
prediction. We are very perplexed.”
In 2005, Riess and members of the SHOES (Supernova H0 for
the Equation of State) Team set out to measure the universe’s expansion
rate with unprecedented accuracy. In the following years, by refining
their techniques, this team shaved down the rate measurement’s
uncertainty to unprecedented levels. Now, with the power of Hubble and
Gaia combined, they have reduced that uncertainty to just 2.2 percent.
Because the Hubble constant is needed to estimate the age of the
universe, the long-sought answer is one of the most important numbers in
cosmology. It is named after astronomer Edwin Hubble, who nearly a
century ago discovered that the universe was uniformly expanding in all
directions—a finding that gave birth to modern cosmology.
Galaxies appear to recede from Earth proportional to their distances,
meaning that the farther away they are, the faster they appear to be
moving away. This is a consequence of expanding space, and not a value
of true space velocity. By measuring the value of the Hubble constant
over time, astronomers can construct a picture of our cosmic evolution,
infer the make-up of the universe, and uncover clues concerning its
ultimate fate.
The two major methods of measuring this number give incompatible
results. One method is direct, building a cosmic “distance ladder” from
measurements of stars in our local universe. The other method uses the
CMB to measure the trajectory of the universe shortly after the Big Bang
and then uses physics to describe the universe and extrapolate to the
present expansion rate. Together, the measurements should provide an
end-to-end test of our basic understanding of the so-called “Standard
Model” of the universe. However, the pieces don’t fit
.
.
Using Hubble and newly released data from Gaia, Riess’ team measured
the present rate of expansion to be 73.5 kilometers (45.6 miles) per
second per megaparsec. This means that for every 3.3 million light-years
farther away a galaxy is from us, it appears to be moving 73.5
kilometers per second faster. However, the Planck results predict the
universe should be expanding today at only 67.0 kilometers (41.6 miles)
per second per megaparsec. As the teams’ measurements have become more
and more precise, the chasm between them has continued to widen, and is
now about 4 times the size of their combined uncertainty.
Over the years, Riess’ team has refined the Hubble constant value by
streamlining and strengthening the “cosmic distance ladder,” used to
measure precise distances to nearby and far-off galaxies. They compared
those distances with the expansion of space, measured by the stretching
of light from nearby galaxies. Using the apparent outward velocity at
each distance, they then calculated the Hubble constant.
To gauge the distances between nearby galaxies, his team used a
special type of star as cosmic yardsticks or milepost markers. These
pulsating stars, called Cepheid variables, brighten and dim at rates
that correspond to their intrinsic brightness. By comparing their
intrinsic brightness with their apparent brightness as seen from Earth,
scientists can calculate their distances.
Gaia further refined this yardstick by geometrically measuring the
distance to 50 Cepheid variables in the Milky Way. These measurements
were combined with precise measurements of their brightnesses from
Hubble. This allowed the astronomers to more accurately calibrate the
Cepheids and then use those seen outside the Milky Way as milepost
markers.
“When you use Cepheids, you need both distance and brightness,”
explained Riess. Hubble provided the information on brightness, and Gaia
provided the parallax information needed to accurately determine the
distances. Parallax is the apparent change in an object’s position due
to a shift in the observer’s point of view. Ancient Greeks first used
this technique to measure the distance from Earth to the Moon.
“Hubble is really amazing as a general-purpose observatory, but Gaia
is the new gold standard for calibrating distance. It is purpose-built
for measuring parallax—this is what it was designed to do,” Stefano
Casertano of Space Telescope Science Institute and a member of the SHOES
Team added. “Gaia brings a new ability to recalibrate all past distance
measures, and it seems to confirm our previous work. We get the same
answer for the Hubble constant if we replace all previous calibrations
of the distance ladder with just the Gaia parallaxes. It’s a crosscheck
between two very powerful and precise observatories.”
The goal of Riess’ team is to work with Gaia to cross the threshold
of refining the Hubble constant to a value of only one percent by the
early 2020s. Meanwhile, astrophysicists will likely continue to grapple
with revisiting their ideas about the physics of the early universe.
The Riess team's latest results are published in the July 12 issue of the Astrophysical Journal.
The Hubble Space Telescope is a project of international cooperation
between NASA and ESA (European Space Agency). NASA's Goddard Space
Flight Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Maryland, conducts
Hubble science operations. STScI is operated for NASA by the Association
of Universities for Research in Astronomy, in Washington, D.C.
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Contacts
Ann Jenkins / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4488 / 410-338-4514
jenkins@stsci.edu/ villard@stsci.edu
Adam Riess
Space Telescope Science Institute, Baltimore, Maryland
410-516-4474
ariess@stsci.edu
Space Telescope Science Institute, Baltimore, Maryland
410-338-4488 / 410-338-4514
jenkins@stsci.edu/ villard@stsci.edu
Adam Riess
Space Telescope Science Institute, Baltimore, Maryland
410-516-4474
ariess@stsci.edu
Source: HubbleSite/News
