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Tuesday, 24 December 2019

The Expanding Universe

The universe now seems to be expanding at a rate even faster than previously thought.
Since first proposed by George Lemaitre and the subsequent confirmation by Edwin Hubble’s observational studies of galactic recession velocities, the expansion of the universe has long been a topic of debate. Improved methods along with differing techniques has continuously yielded discrepancies. For example, techniques utilizing standard candles, in the form of Supernovae Type 1aCepheid variables or Quasars, for nearby observations of the modern universe yields higher values than those found from the distant Cosmic Microwave Background (CMB) observations of the early universe.
Now a team of scientists at the University of California, Davis, have found the highest value yet, suggesting that the universe is expanding at an even greater rate than previously thought.
The team led by Geoff Chen combined new adaptive optics imaging from the Keck Telescope in Mauna Kea along with Hubble Space Telescope imaging of three known quasars. Utilizing the technique of gravitational lensing, as described here, they were able to detect the light from the quasar and infer a value for the Hubble constant. In all three cases they found a value higher than that found from CMB measurements with the combined value being greater than all previous measurements.
“Therein lies the crisis in cosmology …while the Hubble Constant is constant everywhere in space at a given time, it is not constant in time. So, when we are comparing the Hubble Constants that come out of various techniques, we are comparing the early universe (using distant observations) vs. the late, more modern part of the universe (using local, nearby observations)” – Chris Fassnacht, Professor of Physics at UC Davis
The standard model describes the universe as expanding extremely fast, then slowing down and then increasing again, which does not agree with what we are observing. So, either there is a problem with the measurements, or the standard model needs to be modified.

RSF in perspective

The value of the Hubble constant differs between different measuring techniques. As these techniques are essentially observing at different space-time coordinates, it makes sense that this is what is causing the discrepancy. That is, CMB measurements are observations of the early universe and will therefore give us a lower Hubble constant. The standard model may therefore need to be modified with a more unified approach where the Hubble constant – the expansion rate – is modeled as a function of time.

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