Astronomers have made a new measurement of how fast the universe is expanding, using an entirely different kind of star than previous endeavors. The revised
measurement, which comes from NASA's Hubble Space Telescope, falls in
the center of a hotly debated question in astrophysics that may lead to a
new interpretation of the universe's fundamental properties.
Scientists
have known for almost a century that the universe is expanding, meaning
the distance between galaxies across the universe is becoming ever more
vast every second. But exactly how fast space is stretching, a value
known as the Hubble constant, has remained stubbornly elusive.
Now,
University of Chicago professor Wendy Freedman and colleagues have a
new measurement for the rate of expansion in the modern universe,
suggesting the space between galaxies is stretching faster than
scientists would expect. Freedman's is one of several recent studies
that point to a nagging discrepancy between modern expansion
measurements and predictions based on the universe as it was more than
13 billion years ago, as measured by the European Space Agency's Planck
satellite.
As more research points to a discrepancy between
predictions and observations, scientists are considering whether they
may need to come up with a new model for the underlying physics of the
universe in order to explain it.
"The Hubble constant is the
cosmological parameter that sets the absolute scale, size and age of the
universe; it is one of the most direct ways we have of quantifying how
the universe evolves," said Freedman. "The discrepancy that we saw
before has not gone away, but this new evidence suggests that the jury
is still out on whether there is an immediate and compelling reason to
believe that there is something fundamentally flawed in our current
model of the universe.”
In a new paper accepted for publication in The Astrophysical Journal,
Freedman and her team announced a new measurement of the Hubble
constant using a kind of star known as a red giant. Their new
observations, made using Hubble, indicate that the expansion rate for
the nearby universe is just under 70 kilometers per second per
megaparsec (km/sec/Mpc). One parsec is equivalent to 3.26 light-years
distance.
This measurement is slightly smaller than the value of 74 km/sec/Mpc recently reported by the Hubble SH0ES (Supernovae H0
for the Equation of State) team using Cepheid variables, which are
stars that pulse at regular intervals that correspond to their peak
brightness. This team, led by Adam Riess of the Johns Hopkins University
and Space Telescope Science Institute, Baltimore, Maryland, recently
reported refining their observations to the highest precision to date
for their Cepheid distance measurement technique.
A
central challenge in measuring the universe's expansion rate is that it
is very difficult to accurately calculate distances to distant objects.
In
2001, Freedman led a team that used distant stars to make a landmark
measurement of the Hubble constant. The Hubble Space Telescope Key
Project team measured the value using Cepheid variables as distance
markers. Their program concluded that the value of the Hubble constant
for our universe was 72 km/sec/Mpc.
But more recently, scientists
took a very different approach: building a model based on the rippling
structure of light left over from the big bang, which is called the
Cosmic Microwave Background. The Planck measurements allow scientists to
predict how the early universe would likely have evolved into the
expansion rate astronomers can measure today. Scientists calculated a
value of 67.4 km/sec/Mpc, in significant disagreement with the rate of
74.0 km/sec/Mpc measured with Cepheid stars.
Astronomers have
looked for anything that might be causing the mismatch. "Naturally,
questions arise as to whether the discrepancy is coming from some aspect
that astronomers don't yet understand about the stars we're measuring,
or whether our cosmological model of the universe is still incomplete,"
Freedman said. "Or maybe both need to be improved upon."
Freedman's
team sought to check their results by establishing a new and entirely
independent path to the Hubble constant using an entirely different kind
of star.
Certain stars end their lives as a very luminous kind of
star called a red giant, a stage of evolution that our own Sun will
experience billions of years from now. At a certain point, the star
undergoes a catastrophic event called a helium flash, in which the
temperature rises to about 100 million degrees and the structure of the
star is rearranged, which ultimately dramatically decreases its
luminosity.
Astronomers can measure the apparent brightness of the red
giant stars at this stage in different galaxies, and they can use this
as a way to tell their distance.
The Hubble constant is calculated
by comparing distance values to the apparent recessional velocity of
the target galaxies — that is, how fast galaxies seem to be moving away.
The team's calculations give a Hubble constant of 69.8 km/sec/Mpc —
straddling the values derived by the Planck and Riess teams.
"Our
initial thought was that if there's a problem to be resolved between the
Cepheids and the Cosmic Microwave Background, then the red giant method
can be the tie-breaker," said Freedman.
But the results do not
appear to strongly favor one answer over the other say the researchers,
although they align more closely with the Planck results.
NASA's
upcoming mission, the Wide Field Infrared Survey Telescope (WFIRST),
scheduled to launch in the mid-2020s, will enable astronomers to better
explore the value of the Hubble constant across cosmic time. WFIRST,
with its Hubble-like resolution and 100 times greater view of the sky,
will provide a wealth of new Type Ia supernovae, Cepheid variables, and
red giant stars to fundamentally improve distance measurements to
galaxies near and far.
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.
Source: HubbleSite/News
Contact:
Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu
Louise Lerner
University of Chicago, Chicago, Illinois
773-702-8366
louise@uchicago.edu
Related Links:
- The science paper by W. Freedman et al.
- University of Chicago's Release
- Carnegie's Release
- NASA's Hubble portal
- NASA SMD's ScienceCast (Sept. 20, 2017): "Hubble's Contentious Constant"
- Freedman's science paper on astro-pH
