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Study Suggests Space Between Galaxies is Expanding Faster than Expected

Astronomers have used a completely different type of star than used previously to make a revised measurement of the rate at which the universe is expanding.

(Image credit: NASA, ESA, W. Freedman (University of Chicago), ESO, and the Digitized Sky Survey)

The new measurement, from NASA’s Hubble Space Telescope, lies at the core of a hot topic of debate in astrophysics that could offer a new perception about the fundamental properties of the universe.

For nearly 100 years, scientists have been aware of the fact that the universe is expanding, that is, the distance between galaxies throughout the universe is turning out to be ever vaster each second. However, the Hubble constant, or the value of precisely how fast space is expanding, has been indefinable unyieldingly.

Currently, Wendy Freedman, a University of Chicago professor, and her coworkers have made a new measurement for the expansion rate of the modern universe, indicating that the space between galaxies is expanding faster than researchers would anticipate.

Freedman’s is one of numerous latest studies pointing toward a nagging discrepancy between modern measurements of the expansion and predictions based on the universe as it was over 13 billion years earlier, as measured by the Planck satellite of the European Space Agency.

With more and more studies pointing toward a discrepancy between observations and predictions, researchers are examining whether they might have to develop a new model for the fundamental physics behind 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.

Wendy Freedman, Professor, University of Chicago

Freedman added, “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 study accepted for publication in The Astrophysical Journal, Freedman and her colleagues have described a new measurement of the Hubble constant with the help of a type of star called a red giant. Their new observations, using Hubble, show that the expansion rate for the adjacent universe is only under 70 km per second per megaparsec (km/second/Mpc), where 1 parsec is equal to 3.26 light-years.

This value is somewhat less than the value of 74 km/second/Mpc reported recently by the Hubble SH0ES (Supernovae H0 for the Equation of State) group that used Cepheid variables, which are stars pulsing at regular intervals corresponding to their peak brightness.

This group was led by Adam Riess of the Johns Hopkins University and Space Telescope Science Institute, Baltimore, Maryland, and recently reported that their observations were fine-tuned to the maximum precision to date for their Cepheid distance measurement method.

How to Measure Expansion

The main difficulty in measuring the expansion rate of the universe is that it is quite challenging to make precise calculations of distances to distant objects.

In 2001, Freedman led a group of researchers who used distant stars to carry out a landmark measurement of the Hubble constant. The value was measured by the Hubble Space Telescope Key Project team using Cepheid variables as distance markers. It was concluded that the Hubble constant’s value for our universe was 72 km/second/Mpc.

However, more recently, researchers used a quite different strategy: developing a model based on the rippling structure of light remaining from the big bang, known as the Cosmic Microwave Background.

The Planck measurements enable researchers to estimate the way in which the early universe would probably have evolved into the expansion rate astronomers can measure currently. Researchers calculated a value of 67.4 km/second/Mpc, in considerable conflict with the rate of 74.0 km/second/Mpc measured using Cepheid stars.

Astronomers have searched for things that might be the reason for 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. Or maybe both need to be improved upon.

Wendy Freedman, Professor, University of Chicago

Freedman and colleagues intended to verify their results by determining a new and completely independent path to the Hubble constant using a totally different type of star.

The lives of some stars end as a highly luminous type of star known as a red giant, an evolution stage that will be experienced by the Sun billions of years later. At a specific point, the star experiences a catastrophic event known as a helium flash, where the temperature increases to about 100 million degrees and the star’s structure is rearranged, which eventually decreases its luminosity drastically.

Astronomers can measure the evident brightness of the red giant stars at this stage in various galaxies. They can employ this as a method to predict their distance.

Calculation of the Hubble constant is performed by comparing distance values to the evident recessional velocity of the target galaxies, or the rate at which galaxies appear to be moving away. The calculations by the team offer a Hubble constant of 69.8 km/second/Mpc—straddling the values calculated by the Planck and Riess groups.

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.

Wendy Freedman, Professor, University of Chicago

According to the researchers, the results do not seem to explicitly favor one answer over the other, though they align more closely with the Planck results.

The Wide Field Infrared Survey Telescope (WFIRST), which is NASA’s forthcoming mission planned to launch in the mid-2020s, will allow astronomers to better investigate the value of the Hubble constant across cosmic time.

Thanks to its Hubble-like resolution and 100 times greater view of the sky, WFIRST will offer a wealth of new Type Ia supernovae, red giant stars, and Cepheid variables to basically enhance distance measurements to adjacent and faraway galaxies.


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