Tue, 30 Aug 2016
An extra Gigayear for the Universe?
Popular science descriptions of our present understanding of
observational cosmology tend to say that we know the age of the
Universe to be 13.80 gigayears, with an uncertainty of just 0.02
gigayears (20 megayears). But some of the oldest microlensed stars in the Galactic Bulge, within the central
kiloparsec or so of our Galaxy,
have best estimated ages of
about 14.7 gigayears!. In the figure at left, our analysis of the
probability distribution of the most likely age of the oldest of these
stars is shown. The thin curves show probability densities for the
ages of individual stars—several of these peak between about
14.5 and 15 gigayears. The thick curve shows the age of the oldest of
these stars, supposing that we choose the individual star ages randomly
according to their probability distributions. (This includes possible ages much lower
than in the figure; we take the full asymmetric distributions into account.)
So could the Universe be a gigayear older than
is generally thought? The uncertainties are still big, but this is certainly an exciting
prospect for shifting towards a more physically motivated cosmological model.
The more careful descriptions
of the age of the Universe give a caveat—a warning of how or
why the standard estimate might be wrong—the age estimate depends on fitting
observations by using the standard ΛCDM model. Which is the
standard model of cosmology. Meaning that it makes a
nonstandard assumption about gravity. Instead of allowing
space to curve differently in regions where matter collapses into
galaxies versus places where the Universe becomes more empty, which is
what Einstein's general relativity says, the standard model is rigid
(apart from uniform expansion). It
doesn't allow general relativity to apply properly.
Several of us have been working on theoretical tools and
observational analysis to see if we can apply general relativity
better than in the standard model. At least so far, we generally find that doing our
homework tells us that the wouldbe mysterious "dark energy" is really, until or unless
proven otherwise, just a misinterpretation of space recently becoming
negatively curved (on average) as voids and galaxies
have formed during the most recent several gigayears.
This is where the age of the Universe comes in. In our new
paper, arXiv:1608.06004,
my colleagues and I summarise some key numbers that we argue are
needed by any of the "backreaction" models similar to ours, which
allow space to curve as galaxies and voids form, as required by the
Einstein equation of general relativity. These simple constraints show
that by fitting a nodarkenergy flat model (the
Einstein–de Sitter model) at early times, the age of the
Universe should be somewhat less than 17.3 gigayears, and quite likely
somewhat more than the ΛCDM estimate of 13.8 gigayears. So we
looked at published observations of stellar ages, which individually
still have big uncertainties, but together favour the oldest stars
having ages of around 14.7 gigayears. As expected, this is somewhere
in between the two limits of 13.8 and 17.3 gigayears.
So will there be a race between detailed "backreaction" models
versus stellar observers to get tight cosmological predictions of the
age of the Universe versus accurate spectrosopic measurements of the
oldest Galactic stars's ages (which have to be younger than the
Universe, of course!)?
Barely had our paper become public on ArXiv, that we were reminded
by colleagues studying cosmic microwave background (CMB) observations
using the Einstein–de Sitter, nodarkenergy, flat
cosmological model at early times that they also found an age
of the Universe of something like 14.5 gigayears! Figure 4
bottomright of
arXiv:1012.3460 (PRD)
shows our colleagues' estimates of the age of the Universe using the
CMB and type Ia supernovae observations. Their most likely age is
about 14.5 gigayears, give or take about half a gigayear. This is not
so very different from the Galactic Bulge star best estimate! So we
have very different, independent methods tending to give similar
results. The uncertainties are still big. This story is not
closed. But an extra Gigayear for the age of the Universe may be a
clue that helps shift from the precise ΛCDM cosmology to the upcoming
generation of accurate cosmology...
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Sat, 13 Feb 2016
New Windows on the Universe
David Wiltshire from Roy Kerr's institute at the University
of Canterbury describes the
LIGO discovery of a beautiful gravitational
wave signal of a binary black hole merger:

As researchers with the Laser Interferometer
GravitationalWave Observatory (LIGO) announce they
have spotted gravitational waves – ripples in space itself
set off by violent astrophysical events – University of
Canterbury Professor of Physics Dr David Wiltshire discusses what it
means.
The announcement that gravitational waves have been directly
captured for the first time ever, from the collision of two black
holes, opens a new age of astronomy. From now on we will be able to
“listen” to the Universe with “ears” that are
not limited by the electromagnetic spectrum, completely changing our
understanding. It is a moment in history every bit as important as
when Galileo first pointed his telescope at the stars and planets, or
when the first radio, Xray, infrared or gamma ray telescopes were
first turned on by 20th century astronomers.
... more at Reflections on Science.

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Sat, 06 Feb 2016
Inhomogeneous relativistic cosmology ADS links
A mix of some recent and some older articles on inhomogeneous relativistic cosmology—especially cosmological backreaction—is available at:
ADS inhomogeneous cosmology. This list is not complete!
For selecting a subset of the above:  click on the ADS URL above;
 in the ADS interface, Select All Records;
 Get form to query selected articles only;
 enter normal ADS query parameters, e.g. author, year, and/or words in title or abstract;
 Send Query;
The bibcode list is here:
2018arXiv180510455B
2018arXiv180108566B
2018arXiv180108362W
2018MNRAS.473L..46B
2018CQGra..35b4003B
2018AAS...23142005B
2017arXiv171204229T
2017arXiv171204041B
2017arXiv171202967B
2017arXiv171008804B
2017arXiv171002496G
2017arXiv171002451M
2017arXiv171002374D
2017arXiv170906022M
2017arXiv170809143B
2017arXiv170807769F
2017arXiv170801031H
2017arXiv170700191S
2017arXiv170700111Y
2017arXiv170609309A
2017arXiv170607236D
2017arXiv170606179R
2017arXiv170502328S
2017arXiv170408975L
2017arXiv170402810B
2017arXiv170401774C
2017arXiv170400703B
2017arXiv170308809K
2017arXiv170308552S
2017arXiv170304191R
2017arXiv170303572K
2017arXiv170100819S
2017PhRvD..95f3517S
2017JCAP...07..028S
2017A&A...598A.111R
2016arXiv161209309C
2016arXiv161208222B
2016arXiv161203726F
2016arXiv161109275B
2016arXiv161105447M
2016arXiv161103437D
2016arXiv161102139P
2016arXiv161101885S
2016arXiv160904081E
2016arXiv160903724F
2016arXiv160902061C
2016arXiv160806004R
2016arXiv160804403G
2016arXiv160800534M
2016arXiv160800452C
2016arXiv160708797R
2016arXiv160707377K
2016arXiv160705677L
2016arXiv160403503W
2016arXiv160307837T
2016arXiv160301479L
2016arXiv160200302O
2016arXiv160200121R
2016arXiv160107362B
2016arXiv160107356O
2016arXiv160107030S
2016arXiv160100110K
2016PhRvL.116y1301G
2016MNRAS.457.3285M
2016MNRAS.456L..45R
2016JCAP...06..035B
2016JCAP...01..009K
2016IJMPD..2530007B
2016GReGr..48...51B
2015mgm..conf..922K
2015arXiv151209348R
2015arXiv151207869C
2015arXiv151202947O
2015arXiv151105124B
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2015MNRAS.453.2364B
2015MNRAS.448.1660R
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2015CQGra..32u5021B
2015CQGra..32u5013K
2015CQGra..32p5012S
2015CQGra..32m5007V
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2014PhRvD..90b3524K
2014PhRvD..89h9901K
2014PhRvD..89b3520K
2014MNRAS.437.1096R
2014JCAP...12..049C
2014JCAP...05..031B
2014JCAP...02..009B
2014EPJC...74.2786F
2014CQGra..31i5012K
2014CQGra..31h5002K
2014CQGra..31g5021S
2013arXiv1311.5402O
2013arXiv1311.3787W
2013PhRvL.111y1302I
2013PhRvD..88h3529W
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2013GReGr..45.2529R
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2013GReGr..45.1515R
2013CQGra..30w5008B
2013CQGra..30q5006D
2013CQGra..30f5016S
2013CQGra..30f5015S
2013CQGra..30b5002B
2012igrc.book.....P
2012dsu..workE..18S
2012coup.book....1B
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2012arXiv1210.2161L
2012arXiv1206.6164E
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2012PhRvD..85h3528R
2012PhRvD..85h3502I
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2012GReGr..44..353R
2012GReGr..44..239K
2012GReGr..44...81J
2012EPJC...72.2242R
2012CQGra..29p5007B
2012CQGra..29o5001B
2012CQGra..29k5004R
2012ARNPS..62...57B
2011arXiv1110.1828K
2011arXiv1109.2314C
2011arXiv1105.1864E
2011PhRvD..84l3001D
2011PhRvD..84b3510K
2011PhRvD..83h4020G
2011PhRvD..83h3503K
2011PhRvD..83f3506N
2011MNRAS.418L..45B
2011MNRAS.418.2779S
2011MNRAS.413..367S
2011JCos...15.6100W
2011JCAP...10..016E
2011JCAP...10..002P
2011JCAP...05..028N
2011JCAP...05..003M
2011JCAP...03..029U
2011CQGra..28w5002S
2011CQGra..28p5004R
2011CQGra..28p4009K
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2011CQGra..28p4007B
2011CQGra..28p4006W
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2011CQGra..28p4001E
2011CQGra..28p2002B
2010arXiv1012.3084B
2010arXiv1012.3083B
2010arXiv1012.0784R
2010arXiv1003.4020V
2010arXiv1001.0904S
2010PhRvD..82l3528R
2010PhRvD..82b3523W
2010PhRvD..81j3512R
2010PhRvD..81b3501G
2010MNRAS.401..547H
2010JCAP...12..021M
2010JCAP...10..021M
2010JCAP...05..020E
2010JCAP...03..018R
2010JCAP...01..004E
2010IJMPD..19.1915B
2010GReGr..42.2813S
2010GReGr..42.2453K
2010GReGr..42.1399K
2010GReGr..42..567M
2010CQGra..27x5017C
2010CQGra..27q5013R
2010CQGra..27q5001S
2010CQGra..27j9002C
2010CQGra..27j5015K
2010AIPC.1241.1146S
2010AIPC.1241.1074M
2010AIPC.1241..991L
2010AIPC.1241..973K
2010A&A...518A..21C
2009suem.book.....B
2009arXiv0906.1325T
2009PhRvD..80l3512W
2009PhRvD..80h3525C
2009PhRvD..79h4006L
2009PhRvD..79h3011L
2009PhRvD..79b5009S
2009JCAP...02..011R
2009IJMPD..18.2121W
2009GReGr..41.2017B
2009GReGr..41.1585B
2009EAS....36...63R
2009EAS....36...57C
2008wscm.conf..118C
2008mgm..conf.1837C
2008mgm..conf.1831L
2008mgm..conf..700K
2008dmap.conf..565W
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2008IJMPD..17..641W
2008GReGr..40..467B
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2008CQGra..25q5001C
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2007PThPh.117..229K
2007NJPh....9..377W
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2007JCAP...02..019E
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2006tmpg.conf..279K
2006tmgm.meet...80K
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2006igrc.book.....P
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2006PhRvD..74j3507C
2006PhRvD..73b3518H
2006PhLA..354....8B
2006JCAP...11..003R
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2006CQGra..23..817B
2006CQGra..23..235I
2006A&A...454..415B
2005gr.qc.....7057N
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2005MNRAS.362..213B
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2004PhRvL..92n1302H
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2003magr.workE..15K
2003jgrg.conf..157B
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2002nmgm.meet..627K
2002grcg.conf...29B
2002gr.qc....12072Z
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1978AcC.....7..101K
(Last update: 20180610.)
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