Cepheids in UGC 9391
UGC 9391
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Astronomers have used Hubble to measure
the distances to stars in nineteen galaxies more accurately than
previously possible. They found that the Universe is currently expanding
faster than the rate derived from measurements of the Universe shortly
after the Big Bang. If confirmed, this apparent inconsistency may be an
important clue to understanding three of the Universe’s most elusive
components: dark matter, dark energy and neutrinos.
A team of astronomers, led by Nobel Laureate Adam Riess
and using the NASA/ESA Hubble Space Telescope, have discovered that the
Universe is expanding between five and nine percent faster than
previously calculated. This is in clear discrepancy with the rate
predicted from measurements of the infant Universe.
“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,” explains Adam Riess of the Space Telescope Science Institute and the Johns Hopkins University, both in Baltimore, USA.
One possible explanation for this unexpectedly fast expansion of the
Universe is a new type of subatomic particle that may have changed the
balance of energy in the early Universe, so called dark radiation.
The team made the discovery by refining the measurement of how fast the Universe is expanding, a value called the Hubble constant, to unprecedented accuracy, reducing the uncertainty to only 2.4 percent [1].
This new measurement presents a puzzle because it does not agree with
the expansion rate found by looking at the moments shortly after the
Big Bang. Measurements of the afterglow from the Big Bang from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency’s Planck satellite mission yield smaller predictions for the Hubble constant.
Comparing the Universe’s expansion rate as calculated by WMAP and
Planck (for the time after the Big Bang) and Hubble (for our modern
Universe) is like building a bridge, Riess explains: “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. But now the ends are not
quite meeting in the middle and we want to know why.”
This refined determination of the Hubble constant was made possible
by making precise measurements of the distances to both nearby and
faraway galaxies using Hubble [2]. The improved distance measurements were made by streamlining and strengthening the cosmic distance ladder,
which astronomers use to measure accurate distances to galaxies. The
team compared these measured distances with the expansion of space as
measured by the stretching of light from receding galaxies and these two values were then used to calculate the Hubble constant.
The team is continuing to use Hubble with the aim of reducing the
uncertainty in the Hubble constant even further, their goal being to
reach an uncertainty of just 1 percent. Current telescopes such as the European Space Agency’s Gaia satellite, and future telescopes such as the NASA/ESA/CSA James Webb Space Telescope (JWST) and the European Extremely Large Telescope (E-ELT)
could also help astronomers make better measurements of the expansion
rate and lead to a better understanding of our Universe and the laws
that govern it.
Notes
[1] Before Hubble was launched in 1990, 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 10 percent, accomplishing one of
the telescope’s key goals. The new, improved Hubble constant value is
73.02 kilometres per second per Megaparsec (where one Megaparsec is
equivalent to 3.26 million light-years).
[2] For the calibration of relatively
short distances the team observed Cepheid variables. These are pulsating
stars which fade and brighten at rates that are proportional to their
true brightness and this property allows astronomers to determine their
distances. The researchers calibrated the distances to the Cepheids
using a basic geometrical technique called parallax. With Hubble’s
sharp-eyed Wide Field Camera 3 (WFC3),
they extended the parallax measurements further than previously
possible, across the Milky Way galaxy. To get accurate distances to
nearby galaxies, the team then looked for galaxies containing both
Cepheids and Type Ia supernovae. Type Ia supernovae always have the same
intrinsic brightness and are also bright enough to be seen at
relatively large distances. By comparing the observed brightness of both
types of stars in those nearby galaxies, the team could then accurately
measure the true brightness of the supernova. Using this calibrated
rung on the distance ladder the accurate distance to additional 300 type
Ia supernovae in far-flung galaxies was calculated.
More Information
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
The international team of astronomers in this study consists of Adam
G. Riess (John Hopkins University, USA; STScI, USA), Lucas M. Macri
(Texas A&M University, USA), Samantha L. Hoffmann (Texas A&M
University, USA), Dan Scolnic (John Hopkins University, USA; University
of Chicago, USA), Stefano Casertano (STScI, USA), Alexei V. Filippenko
(University of California Berkeley, USA), Brad E. Tucker (University of
California Berkeley, USA; Australian National University, Australia),
Mark J. Reid (Harvard-Smithsonian Center for Astrophysics, USA), David
O. Jones (John Hopkins University, USA), Jeffrey M. Silverman
(University of Texas, USA), Ryan Chornock (Ohio University, USA), Peter
Challis (Harvard-Smithsonian Center for Astrophysics, USA), Wenlong Yuan
(Texas A&M University, USA),and Ryan J. Foley (University of
Illinois at Urbana-Champaign, USA).
Image credit: NASA, ESA
Links
Contacts
Adam Riess
Space Telescope Science Institute
Baltimore, USA
Tel: +1 410 516 4474
Email: ariess@stsci.edu
Mathias Jäger
ESA/Hubble, Public Information Officer
Garching bei München, Germany
Cell: +49 176 62397500
Email: mjaeger@partner.eso.org
