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Science Coronavirus Coverage U. Travel A road trip in Burgundy reveals far more than fine wine. Things were getting surd. Adam and I then went out to dinner and we were pretty perplexed, because in what we had seen up to this point, the cepheids and TRGBs were in really good agreement.
They soon homed in on the key change in the paper: a new way of measuring the effects of dust when gauging the intrinsic brightness of TRGBs — the first rung of the cosmic distance ladder. One of the all-time greatest cosmological discoveries, cosmic expansion implies that the universe has a finite age.
Big, bright cepheids pulsate more slowly than small, dim ones just as a big accordion is harder to compress than a tiny one. And so, from the pulsations of a distant cepheid, you can read off how intrinsically bright it is. Hubble then used cepheids to deduce the distances to nearby galaxies, which, plotted against their speeds, revealed cosmic expansion. Hubble overestimated the rate as kilometers per second per megaparsec, but the number dropped as cosmologists used cepheids to calibrate evermore accurate cosmic distance ladders.
His rivals claimed a value around , based on different astronomical observations. To build a distance ladder, you start by calibrating the distance to stars of known luminosity, such as cepheids. These standard candles can be used to gauge the distances to fainter cepheids in farther-away galaxies. Crowding by other stars can make them look brighter and thus closer.
Furthermore, even supposed standard-candle stars have inherent variations due to age and metallicity that must be corrected for. Freedman devised new methods to deal with many sources of systematic error. The H 0 value of 72 that her team published in split the difference in the versus debate. She was gracious and he softened. In its analysis, Planck found H 0 to be Tommaso Treu , one of the founders of H0LiCOW and a professor at the University of California, Los Angeles, had dreamed ever since his student days in Pisa of measuring the Hubble constant using time-delay cosmography — a method that skips the rungs of the cosmic distance ladder altogether.
Instead, you directly determine the distance to quasars — the flickering, glowing centers of faraway galaxies — by painstakingly measuring the time delay between different images of a quasar that form as its light bends around intervening matter.
But while Treu and his colleagues were collecting quasar data, Freedman, Madore and their graduate students and postdocs were pivoting to tip-of-the-red-giant-branch stars. Whereas cepheids are young and found in the crowded, dusty centers of galaxies, TRGBs are old and reside in clean galactic outskirts. Earlier this year, he, Lisa Randall of Harvard University, and others proposed a possible solution to the Hubble constant tension.
Their idea — a new, short-lived field of repulsive energy in the early universe — would speed up cosmic expansion, matching predictions to observations, though this and all other proposed fixes strike experts as a bit contrived. He later said he was half kidding. Here I am, stuck in the middle with you.
Another curveball came before lunch. Mark Reid of the Harvard-Smithsonian Center for Astrophysics presented new measurements of four masers — laserlike effects in galaxies that can be used to determine distances — that he had performed in the preceding weeks. Combined, the masers pegged H 0 at Adam Riess took a picture of the slide. When I spoke with Riess during the midday break, he seemed overwhelmed by all the new measurements.
But its nature remains a mystery. An extra dose of dark energy in the early universe, dubbed early dark energy , could reconcile the conflicting values of the Hubble constant. Lisa Randall, a particle physicist and cosmologist at Harvard University, has proposed ideas for what could be speeding up cosmic expansion. Each of these additions to the standard model takes a different mathematical form — in some, the density of dark energy oscillates, or rocks, while in others it rolls down from a high value to zero.
But in all cases, the early dark energy must disappear after a few hundred thousand years, during an epoch known as recombination. Alongside early dark energy, theorists have put forward other exotic forms of dark energy — such as quintessence and phantom dark energy — that also change as the universe ages.
While these extensions to the standard model relieve the Hubble tension, they are regarded by many cosmologists as fine-tuned — opportune mathematical additions that have no clear justification.
For example, most cosmologists think space exponentially expanded at the start of the Big Bang during a period known as inflation, which was driven by a different kind of dark energy than the one that exists today. But in a preprint submitted to Physical Review D in March, Barker and three co-authors acknowledge that much more analysis is needed to see if the model can describe not only how the universe expands but also how structures like galaxies and clusters evolved.
With contemporary telescopes offering a glut of impressively precise data on such structures, devising a theory that matches all the observations is no mean feat. Even with the extra freedom, most of the nonstandard models only reduce the Hubble tension rather than eliminating it. In the coming years, the Euclid telescope and others will meticulously map how gravity and dark energy have shaped cosmic evolution.
Meanwhile, gravitational waves emitted from colliding neutron stars offer a new way to measure the Hubble constant. The new data will rule out some of these novel solutions to the Hubble tension, but new cracks in the standard model may appear. For now, many cosmologists are loath to complicate the model when it otherwise works so well.
She added that even if the Hubble tension turns out to be nothing more than an accumulation of errors, this search for new physics may not be in vain.
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