A
perfect Einstein ring, or gravitational lens. The red glow is
from dust in a distant galaxy.
ALMA
(NRAO/ESO/NAOJ); B. Saxton NRAO/AUI/NSF; NASA/ESA Hubble, T.
Hunter (NRAO)
Albert Einstein’s general theory of relativity predicts so much
about the universe at large, including the existence of
gravitational lenses or “Einstein rings.”
At the same time, however, Einstein’s famous equations struggle
to fully explain such objects.
While general relativity says a strong source of gravity —
like the sun — will warp the fabric of space, bend light
coming from a distant object, and magnify it to an observer, very
big objects like
galaxies and galaxy clusters make gravitational lenses that
are theoretically too strong (like the one above). General
relativity also can’t fully explain the spinning motions of
galaxies and their stars.
That’s why most physicists think as much as 80% of the mass in
the universe is dark
matter: an invisible source of matter, and its resulting
gravitational force, that fills the gap. They think dark matter
might be made of
hard-to-detect particles, or perhaps an
unfathomable number of
tiny black holes. But we have yet to find smoking-gun
evidence of either.
However, a contentious theory by Erik Verlinde at the University
of Amsterdam suggests dark matter may not be matter at all.
What’s more, astronomers say his idea “is remarkable” in its
ability to explain the behavior of more than 33,000 galaxies that
they studied.
“This does not mean we can completely exclude dark matter,
because there are still many observations that Verlinde’s cannot
yet explain,” study leader and physicist
Margot Brouwer said in a YouTube video
about the research. “However it is a very exciting and promising
first step.”
A new theory of gravity?
Pablo
Carlos Budassi
Called “emergent
gravity,” or EG, Verlinde’s idea was first widely publicized
by the New York Times in 2010. However, it took him 6 years to
craft into a more testable (though not-yet-peer-reviewed) paper
published on arXiv in
November 2016.
Emergent gravity borrows from the very tiny (and very weird)
world of quantum mechanics to suggest that gravity is really a
“dark” gravitational force, though more like a natural side
effect of the fabric of space.
You might think of it as the outcome of a spacetime tug-of-war.
On the one hand, matter locally warps the fabric of space. On the
other, a powerful and as-yet-unexplained force of nature, called
“dark
energy,” is speeding up the expansion of space and the
edge
of the universe in all directions. (But don’t worry, we may
not go through a “big
rip” until at least
2.8 billion years from now.)
Verlinde suggests the fabric of space has a kind of “elastic
memory” for visible matter against expansion, “which can only
relax very slowly” — a friction that, with large pockets of
matter, generates a “dark” gravitational force at large
distances.
Put another way, gravity may be another way that nature tries to
fill a void with chaos, much like air rushing to fill a vacuum,
or the heat of your body escaping into the space around you — no
exotic, invisible, force-carrying particles required.
“In our view this undercuts the common assumption that the laws
of gravity should stay as they are, and hence it removes the
rationale of the dark matter hypothesis,” Verlinde wrote in his
most recent paper. “Indeed, we have argued that the observed dark
matter phenomena are a remnant, a memory effect, of the emergence
of spacetime together with the ordinary matter in it.”
If you’re feeling confused by all this, you’re not alone: Dennis
Overbye wrote for the
New York Times in 2010 that “[s]ome of the best physicists in
the world say they don’t understand Dr. Verlinde’s paper, and
many are outright skeptical.”
However, a team of astronomers recently ran Verlinde’s equations
through a limited test — and they appeared to check out.
Emergent gravity clears its first hurdle
A
large pocket of gravitational lenses, as seen by the Hubble Space
Telescope.
NASA/ESA/Pontificia
Universidad Católica de Chile
Brouwer and her colleagues tested emergent gravity by studying
warped space around 33,613 galaxies.
Specifically, they looked at gravitational lenses caused by the
galaxies, and how the background objects behind them were
distorted.
“These bent images allow us to reconstruct the gravitational
force around foreground galaxies up to a distance that is 100
times larger than the galaxies themselves,” Brouwer said of her
team’s research, which was published December 11 in the British journal
Monthly Notices of the Royal Astronomical Society.
“Usually we explain this gravity by assuming that each galaxy has
a dark matter cloud of a certain mass,” she said. “This time we
also compared our data to the new theory of gravity by Erik
Verlinde.”
Brouwer said Verlinde’s equations could explain gravity’s
distribution “without introducing any free parameters or
invisible particles.” Translation: No dark matter required.
But the 22 authors of that study (none of whom include Verlinde)
are careful to point out that dark matter is far from a dead
idea.
“Although [emergent gravity’s] performance is remarkable, this
study is only a first step,” they wrote. “Further advancements on
both the theoretical framework and observational tests of EG are
needed before it can be considered a fully developed and solidly
tested theory.”
Timothy
Brandt, an astrophysicist at the Institute for Advanced Study
who’s studied dark matter but wasn’t involved in any of the
studies, told Business Insider in an email that — new evidence
aside — Verlinde’s concept leaves him with more questions than
answers.
Brandt wondered, for example, if emergent gravity can also
explain “the
LIGO results, which perfectly match [general relativity],”
evidence of the “dark matter content
of dwarf galaxies,” and the leftover energy of the universe’s
formation (called the
cosmic background radiation).
Given all of the evidence for general relativity, Brandt said he
“would bet pretty heavily against” emergent gravity’s replacing
it.
Even if Verlinde’s idea turns out to fail future tests, physics
still needs to find a way to solve its biggest problem: how to
unite Einstein’s general relativity (the physics of the very big)
with quantum mechanics (the physics of the very small) into a
so-called “theory
of everything.”
“At large scales, it seems, gravity just doesn’t behave the way
Einstein’s theory predicts,” Verlinde
told Astronomy Now in November.
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