Science & Technology

The Universe is expanding more rapidly than previously believed

This is a ground-based telescope’s view of the Large Magellanic Cloud, a satellite galaxy of our Milky Way. The inset image, taken by the Hubble Space Telescope, reveals one of many star clusters scattered throughout the dwarf galaxy. ( NASA, ESA, Adam Riess, and Palomar Digitized Sky Survey)
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New findings from the Hubble space telescope indicate that the Universe is expanding at a greater rate than previously believed.

Astronomers believe that new measurements from NASA’s Hubble Space Telescope confirm that the Universe is expanding about 9% faster than expected based on its trajectory seen shortly after the big bang.

This means that the Hubble constant (H0) — the measure of the current expansion rate of the Universe, named after Edwin Hubble, the man who first observed said expansion — needs adjustment from its current figure of 
~2 X 10-¹⁸ s-¹.

Adam Riess, Bloomberg Distinguished Professor of Physics and Astronomy at The Johns Hopkins University, Nobel Laureate, says of the disparity between old calculations and these new findings: “This mismatch has been growing and has now reached a point that is really impossible to dismiss as a fluke. This is not what we expected.”

The new measurements — published April 25 in the Astrophysical Journal Letters — reduce the chances that the disparity is an accident from 1 in 3,000 to only 1 in 100,000 and suggest that new physics may be needed to better understand the cosmos.

This illustration shows the three basic steps astronomers use to calculate how fast the universe expands over time, a value called the Hubble constant. All the steps involve building a strong “cosmic distance ladder,” by starting with measuring accurate distances to nearby galaxies and then moving to galaxies farther and farther away. This “ladder” is a series of measurements of different kinds of astronomical objects with an intrinsic brightness that researchers can use to calculate distances ( NASA, ESA, and A. Feild (STScI))

In this study — led by Ries — he and his team, SH0ES (Supernovae, H0, for the Equation of State), analyzed light from 70 stars in our neighbouring galaxy, the Large Magellanic Cloud. SH0ES used a new method that allowed for capturing quick images of these stars — called Cepheid variables, which brighten and dim at constant and predictable rates. The regularity of this brightening and dimming makes Cephid variables ideal to measure nearby intergalactic distances.

The usual method for measuring the stars is incredibly time-consuming; the Hubble can only observe one star for every 90-minute orbit around Earth. Using their new method called DASH (Drift And Shift), the team used Hubble as a ‘point-and-shoot’ camera to look at groups of Cepheids. This ‘snapshot’ approach the team to observe a dozen Cepheids in the time it would normally take to observe just one.

This new data allowed Riess and SH0ES to strengthen the foundation of the cosmic distance ladder — the scale of measurement techniques astronomers use to determine ever increasing distances within the Universe and calculate the Hubble constant.

Combining their Hubble measurements with another set of observations, made by the Araucaria Project — a collaboration between astronomers from institutions in Chile, the U.S., and Europe — the group made distance measurements to the Large Magellanic Cloud by observing the dimming of light as one star passes in front of its partner in eclipsing binary-star systems.

The combined measurements helped the SH0ES team refine the Cepheids’ true brightness. Taking this more accurate result, the team then ‘tightened the bolts’ of the rest of the distance ladder that uses exploding stars called supernovae to extend deeper into space.

As the team’s measurements have become more precise — their calculation of the Hubble constant remained at odds with the expected value derived from observations of the early universe’s expansion by the European Space Agency’s Planck satellite based on conditions Planck observed 380,000 years after the Big Bang.

This is a ground-based telescope’s view of the Large Magellanic Cloud, a satellite galaxy of our Milky Way. The inset image, taken by the Hubble Space Telescope, reveals one of many star clusters scattered throughout the dwarf galaxy. ( NASA, ESA, Adam Riess, and Palomar Digitized Sky Survey)

Continuing to explain the significance of this mismatch, Riess says: “This is not just two experiments disagreeing. We are measuring something fundamentally different.

“One is a measurement of how fast the universe is expanding today, as we see it. The other is a prediction based on the physics of the early universe and on measurements of how fast it ought to be expanding.”

While Riess doesn’t have an answer as to exactly why the discrepancy exists, he and the SH0ES team will continue to fine-tune the Hubble constant, with the goal of reducing the uncertainty to 1%. These most recent measurements brought the uncertainty in the rate of expansion down from 10% in 2001 to 5% in 2009 and now to 1.9% in the present study.

Riess concludes by pointing out the significance for science and our understanding of the Universe: “If these values don’t agree, there becomes a very strong likelihood that we’re missing something in the cosmological model that connects the two eras.”

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