One of the biggest
mysteries in science – dark energy – doesn’t actually exist, according to
researchers looking to solve the riddle of how the Universe is expanding.
For the past 100 years, physicists have
generally assumed that the cosmos is growing equally in all directions. They
employed the concept of dark energy as a placeholder to explain unknown physics
they couldn’t understand, but the contentious theory has always had its
problems.
Now a team of physicists and astronomers
at the University of Canterbury in Christchurch, New Zealand are challenging
the status quo, using improved analysis of supernovae light curves to show that
the Universe is expanding in a more varied, “lumpier” way.
The new evidence supports the
“timescape” model of cosmic expansion, which doesn’t have a need for dark
energy because the differences in stretching light aren’t the result of an
accelerating Universe but instead a consequence of how we calibrate time and
distance.
It takes into account that gravity slows
time, so an ideal clock in empty space ticks faster than inside a galaxy.
The model suggests that a clock in the
Milky Way would be about 35 per cent slower than the same one at an average
position in large cosmic voids, meaning billions more years would have passed
in voids. This would in turn allow more expansion of space, making it seem like
the expansion is getting faster when such vast empty voids grow to dominate the
Universe.
Professor David Wiltshire, who led the
study, said: “Our findings show that we do not need dark energy to explain why
the Universe appears to expand at an accelerating rate.
“Dark energy is a misidentification of
variations in the kinetic energy of expansion, which is not uniform in a
Universe as lumpy as the one we actually live in.”
He added: “The research provides
compelling evidence that may resolve some of the key questions around the
quirks of our expanding cosmos.
“With new data, the Universe’s biggest
mystery could be settled by the end of the decade.”
The new analysis has been published in
the journal Monthly
Notices of the Royal Astronomical Society Letters.
Dark energy is commonly thought to be a
weak anti-gravity force which acts independently of matter and makes up around
two thirds of the mass-energy density of the Universe.
The standard Lambda Cold Dark Matter (ΛCDM) model of the Universe
requires dark energy to explain the observed acceleration in the rate at which
the cosmos is expanding.
Scientists base this conclusion on
measurements of the distances to supernova explosions in distant galaxies,
which appear to be farther away than they should be if the Universe’s expansion
were not accelerating.
However, the present expansion rate of
the Universe is increasingly being challenged by new observations.
Firstly, evidence from the afterglow of
the Big Bang – known as the Cosmic Microwave Background (CMB) – shows the
expansion of the early Universe is at odds with current expansion, an anomaly
known as the “Hubble tension”.
In addition, recent analysis of new high
precision data by the Dark Energy Spectroscopic Instrument (DESI) has found
that the ΛCDM model does not fit as well as models in which dark energy is “evolving” over time, rather than remaining
constant.
Both the Hubble tension and the
surprises revealed by DESI are difficult to resolve in models which use a
simplified 100-year-old cosmic expansion law – Friedmann’s equation.
This assumes that, on average, the
Universe expands uniformly – as if all cosmic structures could be put through a
blender to make a featureless soup, with no complicating structure. However,
the present Universe actually contains a complex cosmic web of galaxy clusters
in sheets and filaments that surround and thread vast empty voids.
Professor Wiltshire added: “We now have
so much data that in the 21st century we can finally answer the question – how
and why does a simple average expansion law emerge from complexity?
“A simple expansion law consistent with
Einstein’s general relativity does not have to obey Friedmann’s equation.”
The researchers say that the European
Space Agency’s Euclid satellite, which was launched in July 2023, has the power
to test and distinguish the Friedmann equation from the timescape alternative.
However, this will require at least 1,000 independent high quality supernovae
observations.
When the proposed timescape model was
last tested in 2017 the analysis suggested it was only a slightly better fit
than the ΛCDM as an explanation for cosmic expansion, so the Christchurch team
worked closely with the Pantheon+ collaboration
team who had painstakingly produced a catalogue of 1,535 distinct supernovae.
They say the new data now provides “very
strong evidence” for timescape. It may also point to a compelling resolution of
the Hubble tension and other anomalies related to the expansion of the
Universe.
Further observations from Euclid and the
Nancy Grace Roman Space Telescope are needed to bolster support for the
timescape model, the researchers say, with the race now on to use this wealth
of new data to reveal the true nature of cosmic expansion and dark energy.
Source: https://ras.ac.uk/news-and-press/research-highlights/dark-energy-doesnt-exist-so-cant-be-pushing-lumpy-universe-apart
Source: Dark
energy ‘doesn’t exist’ so can’t be pushing ‘lumpy’ Universe apart – study –
Scents of Science
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