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A new estimate of the Hubble constant – the rate at which the universe is expanding – is baffling many of the finest minds in the cosmology community
[...] "The fact the universe is expanding is really one of the most powerful ways we have to determine the composition of the universe, the age of the universe and the fate of the universe," said Professor Adam Riess, at the Space Telescope Science Institute in Baltimore, Maryland, who led the latest analysis. "The Hubble constant quantifies all that into one number."
In an expanding universe, the further away a star or galaxy is, the quicker it is receding. Hubble's constant – proposed by Edwin Hubble in the 1920s – reveals by how much.
So one approach to measuring it is by observing the redshifts of bright supernovae, whose light is stretched as the very space it is travelling through expands. A challenge, though, is pinpointing the exact distance of these stars.
Riess, who shared the 2011 Nobel Prize for Physics for providing evidence that the expansion of the universe is accelerating, is part of a team focussed on developing ultra-precise methods for measuring distances.
The latest Gaia observations have advanced this effort by identifying dozens of new Cepheid stars, which have the special feature that their light flickers at a rate that is directly linked to their brightness at source. So through observing the pulsations of these so-called standard candles, scientists can work out their original luminosity and, therefore, how far away they and their native galaxies are.
The new data puts the Hubble constant at 73, which translates to galaxies moving away from us 73km per second faster for each additional megaparsec of distance between us and them (a megaparsec is about 3.3m light-years).
However, a separate estimate of Hubble comes from observations of the Cosmic Microwave Background, relic radiation that allows scientists to calculate how quickly the universe was expanding 300,000 years after the big bang.
"The cosmic microwave background is the light that is the furthest away from us that we can see," said Riess. "It's been travelling for 13.7bn years... and it's telling us how fast the universe was expanding when the universe was a baby."
Scientists then use the cosmic equivalent of a child growth chart (a computational model that roughly describes the age and contents of the universe and the laws of physics) to predict how fast the universe should be expanding today. This gives a Hubble value of 67.
Until recently, scientists had hoped that as measurements became more precise, this mismatch would narrow, but instead the difference has widened and the latest calculation gives a chance of only 1 in 7,000 of the discrepancy being down to chance. "If this continues to hold up we may be dealing with what we call new physics of the universe," said Riess.
[...] The crisis in cosmology, as it was described a meeting of the American Physical Society last month, could soon be resolved through new measurements of the Hubble constant based on gravitational wave observations by the Ligo collaboration. "Within the next five years, we'll probably nail this," said [John] Peacock [professor of cosmology at the University of Edinburgh].
(Score: 3, Interesting) by stormwyrm on Friday May 11 2018, @11:32AM (4 children)
One possible explanation for this discrepancy in measurements of the Hubble Constant is that the Milky Way and most of the nearby galaxies are actually inside a big void [soylentnews.org] (the KBC void). So that means if you try to measure the Hubble Constant by observing relatively nearby galaxies also inside the void, you'll get a higher estimate of the Hubble Constant, since these galaxies will in general be receding from us faster, because they'd be experiencing extra acceleration from the gravity of the mass at the edge of the void. Cepheid variables can only be seen in relatively nearby galaxies, so if the hypothesised void is real, measuring the HC that way will yield a higher value. But if you observed something at the edge of the void or beyond (> ~1 billion light years), like the Cosmic Microwave Background or supernovae in very distant galaxies, you might get the smaller value of the Hubble Constant. I don't know if anyone has tried to measure the Hubble Constant by using observations of only extremely distant supernovae that should be beyond the edge of the KBC void, but if they did get the same lower value of the Constant as we see when trying to measure it with the CMB, then well, that's pretty good evidence of a Hubble bubble [wikipedia.org].
Numquam ponenda est pluralitas sine necessitate.
(Score: 1, Interesting) by Anonymous Coward on Friday May 11 2018, @12:44PM (2 children)
Except that the gravitational effect of a hollow sphere on masses inside it is exactly zero. So you don't get acceleration towards the edge.
The reason is that while the near edge is nearer, the far edge has more mass in the same solid angle. While the attraction per mass goes like 1/r², the mass per solid angle goes like r², so the product is constant, and thus the forces from both sides cancel exactly.
(Score: 2) by hendrikboom on Friday May 11 2018, @02:07PM (1 child)
The boundary of the void may not be a sphere. It may not even be evenly populated.
(Score: 2) by maxwell demon on Friday May 11 2018, @02:47PM
But then you'll get an inhomogeneous acceleration towards specific places (like the Great Attractor [wikipedia.org]), not a homogeneous acceleration towards the edge. Of course such inhomogeneous acceleration can be identified and calculated out of the equation (guess how they found the Great Attractor?).
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by arulatas on Friday May 11 2018, @02:04PM
I believe you mispronounce Hubba Bubba https://en.wikipedia.org/wiki/Hubba_Bubba/ [wikipedia.org]
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