- 4460
- DARK ENERGY
and DARK MATTER?
- 20th-century astronomers
discovered the first of two invisible continents while observing the motions of
stars and galaxies. For outer stars to whip around the center of a galaxy as
quickly as they do they must be held by the gravity of something invisible. The
galaxy’s bright spiral would have to be a small seed sitting in the center of
an unseen cloud of “dark matter.”
-
---------------------------- 4460
- DARK ENERGY
and DARK MATTER?
-
- (
See Review 4459 with Einstein's Theory of Gravity? )
-
- Through increasingly precise observations,
astrophysicists have learned that all the stars and galaxies in the universe
are a bit like city lights viewed from space; they imply the existence of vast,
unseen continents below.
-
- Further evidence for this nonluminous form
of matter appeared when physicists began to scrutinize ancient light from the
early universe. They discerned ripples set off by a struggle between dark
matter and visible radiation. Today, astronomers measure the distribution of
dark matter from how it distorts light
-
- Two teams of cosmologists simultaneously
stumbled upon the second dark continent in the 1990s. The movement of distant
supernovas revealed that the expansion of the universe was picking up speed.
Some sort of “dark energy” was working against gravity to drive galaxies apart
faster and faster.
-
- Theorists immediately identified the prime
suspect: a tiny amount of energy intrinsic to the vacuum of space. Albert
Einstein had long ago considered this possibility, dubbing it a “cosmological
constant.” Such energy would have had a negligible effect early on when the
universe was small, but its effect, repulsion, would grow with the size of the
universe.
-
- In the very long run, the increasingly
rapid expansion is expected to dilute visible and dark matter to imperceptible
levels, leaving an explosively expanding universe made up almost exclusively of
dark energy.
-
- These developments culminated in an
overarching cosmological theory known as the “Lambda-CDM model”, a set of
ingredients and equations describing the evolution of the cosmos. Lambda is a Greek symbol referring to dark energy,
which currently accounts for 70% of the universe’s energy. CDM stands for “cold
dark matter,” which makes up 25% of the cosmos’s energy (via Einstein’s
equivalence of mass and energy). Visible matter, such as atoms , the last 5% of
the cosmic pie chart, doesn’t make it
into the theory’s name but is included in its equations.
-
- In labs across the globe, physicists have
tried to identify the elusive particle, or particles, that seem to be holding
galaxies together. The hope is that dark matter isn’t perfectly dark, and that
it might occasionally light up an appropriately tuned detector.
-
- Increasingly heroic efforts to detect one
promising candidate, a hypothetical particle dubbed the “WIMP”, have now gone
on for decades. But the eureka moment hasn’t come, so in recent years the
search has expanded to cover a wider range of candidate particles.
-
- An alternative possibility is that dark
matter is an illusion because gravity has some quirks we don’t fully
understand. In 2020, theoretical physicists managed to bend Einstein’s theory
of gravity to produce a few of the observed effects of dark matter. They pulled
it off by adding a carefully crafted field, which they described as “dark
dust,” that acted a lot like matter.
-
- While there are many plausible ideas for what
dark matter might be, dark energy has proved more troubling to the foundations
of theoretical physics. Its tiny
density is an especially huge mystery. A naïve estimate of the vacuum energy
based on adding up the base-level energies of all the quantum fields that fill
space gives a number that’s gigantic compared to the tiny source of cosmic
acceleration, a mismatch sometimes referred to as the worst prediction in
physics.
-
- Some physicists suspect, our universe is
one bubble in a vast multiverse, where each bubble has a randomly determined
vacuum energy. We live in a bubble with a minuscule vacuum energy because if
its value were any larger, as Nobel laureate Steven Weinberg pointed out in
1987, the universe would have expanded too quickly for galaxies to form and
(therefore) for life to arise.
-
- Weinberg’s argument came to weigh heavily
on many physicists. It places a major feature of our reality beyond the reach
of calculation. Theorists have made
some attempts to turn the multiverse hypothesis into a predictive theory, but
multiverse abhorrence largely continues today.
-
- Meanwhile, observers have plowed ahead,
tracking the effect of dark energy throughout cosmic history in ever-increasing
detail. Their surveys of the cosmos have usually confirmed Lambda-CDM and the
idea of dark energy as a cosmological constant.
-
- A team announced a tantalizing new clue to
dark energy’s nature. Analyzing thousands of supernovas together with subtle
ripples in the distribution of 6 million galaxies, the “Dark Energy
Spectroscopic Instrument” (DESI) collaboration found a hint that dark energy
might be weakening over time.
-
- The anomaly could be a statistical fluke,
or indicative of a misunderstanding about the astrophysics of supernovas. If
so, further data from the DESI team should resolve the situation in the next
few years.
-
- But if that hint does become a discovery,
its implications will be profound. To some string theorists, it’s natural to
expect that our universe’s vacuum energy might be dropping, since universes
with constant positive vacuum energies appear difficult to construct in string
theory.
-
- Others, however, stress that it would only
create more headaches. Physicists would have to explain not only why the vacuum
energy is improbably tiny, but also why it's changing improbably slowly. Is dark energy really weakening?
-
May 7, 2024 DARK
ENERGY and DARK
MATTER? 4460
------------------------------------------------------------------------------------------
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------ “Jim Detrick” -----------
--------------------- --- Tuesday, May 7, 2024
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