Astrophysicists reveal the nature of dark matter through the study of
crinkles in spacetime

Date:
April 25, 2023
Source:
The University of Hong Kong
Summary:
Astrophysicists have provided the most direct evidence yet that
Dark Matter does not constitute ultramassive particles as is
commonly thought but instead comprises particles so light that
they travel through space like waves. Their work resolves an
outstanding problem in astrophysics first raised two decades ago:
why do models that adopt ultramassive Dark Matter particles fail
to correctly predict the observed positions and the brightness of
multiple images of the same galaxy created by gravitational lensing?

Facebook Twitter Pinterest LinkedIN Email

==========================================================================
FULL STORY ==========================================================================
Most of the matter in the universe, amounting to a staggering 85%
by mass, cannot be observed and consists of particles not accounted
for by the Standard Model of Particle Physics (see remark 1). These
particles are known as Dark Matter, and their existence can be inferred
from their gravitational effects on light from distant galaxies. Finding
the particle that makes up Dark Matter is an urgent problem in modern
physics, as it dominates the mass and, therefore, the gravity of galaxies
-- solving this mystery can lead to new physics beyond the Standard Model.

While some theoretical models propose the existence of ultramassive
particles as a possible candidate for Dark Matter, others suggest
ultralight particles. A team of astrophysicists led by Alfred AMRUTH, a
PhD student in the team of Dr Jeremy LIM of the Department of Physics at
The University of Hong Kong (HKU), collaborating with Professor George
SMOOT, a Nobel Laureate in Physics from the Hong Kong University of
Science and Technology (HKUST) and Dr Razieh EMAMI, a Research Associate
at the Center for Astrophysics | Harvard & Smithsonian (CFA), has provided
the most direct evidence yet that Dark Matter does not constitute
ultramassive particles as is commonly thought but instead comprises
particles so light that they travel through space like waves. Their work resolves an outstanding problem in astrophysics first raised two decades
ago: why do models that adopt ultramassive Dark Matter particles fail to correctly predict the observed positions and the brightness of multiple
images of the same galaxy created by gravitational lensing? The research findings were recently published in Nature Astronomy.

Dark Matter does not emit, absorb or reflect light, which makes it
difficult to observe using traditional astronomical techniques. Today,
the most powerful tool scientists have for studying Dark Matter is through gravitational lensing, a phenomenon predicted by Albert Einstein in his
theory of General Relativity.

In this theory, mass causes spacetime to curve, creating the appearance
that light bends around massive objects such as stars, galaxies, or groups
of galaxies. By observing this bending of light, scientists can infer
the presence and distribution of Dark Matter -- and, as demonstrated in
this study, the nature of Dark Matter itself.

When the foreground lensing object and the background lensed object --
both constituting individual galaxies in the illustration -- are closely aligned, multiple images of the same background object can be seen in the
sky. The positions and brightness of the multiply-lensed images depend
on the distribution of Dark Matter in the foreground lensing object,
thus providing an especially powerful probe of Dark Matter.

Another assumption of the nature of Dark Matter In the 1970s, after the existence of Dark Matter was firmly established, hypothetical particles referred to as Weakly Interacting Massive Particles (WIMPs) were proposed
as candidates for Dark Matter. These WIMPs were thought to be ultramassive
-- more than at least ten times as massive as a proton - - and interact
with other matter only through the weak nuclear force. These particles
emerge from Supersymmetry theories, developed to fill deficiencies in the Standard Model, and have since been widely advocated as the most likely candidate for Dark Matter. However, for the past two decades, adopting ultramassive particles for Dark Matter, astrophysicists have struggled
to correctly reproduce the positions and brightness of multiply-lensed
images. In these studies, the density of Dark Matter is assumed to
decrease smoothly outwards from the centres of galaxies in accordance
with theoretical simulations employing ultramassive particles.

Beginning also in the 1970s, but in dramatic contrast to WIMPs, versions
of theories that seek to rectify deficiencies in the Standard Model,
or those (e.g., String Theory) that seek to unify the four fundamental
forces of nature (the three in the Standard Model, along with gravity), advocate the existence of ultralight particles. Referred to as axions,
these hypothetical particles are predicted to be far less massive than
even the lightest particles in the Standard Model and constitute an
alternative candidate for Dark Matter.

According to the theory of Quantum Mechanics, ultralight particles
travel through space as waves, interfering with each other in such large numbers as to create random fluctuations in density. These random density fluctuations in Dark Matter give rise to crinkles in spacetime. As might
be expected, the different patterns of spacetime around galaxies depending
on whether Dark Matter constitutes ultramassive or ultralight particles
-- smooth versus crinkly -- ought to give rise to different positions
and brightness for multiply-lensed images of background galaxies.

In work led by Alfred AMRUTH, a PhD student in Dr Jeremy LIM's
team at HKU, astrophysicists have for the first time computed how gravitationally-lensed images generated by galaxies incorporating
ultralight Dark Matter particles differ from those incorporating
ultramassive Dark Matter particles.

Their research has shown that the general level of disagreement found
between the observed and predicted positions as well as the brightness of multiply- lensed images generated by models incorporating ultramassive
Dark Matter can be resolved by adopting models incorporating ultralight
Dark Matter particles.

Moreover, they demonstrate that models incorporating ultralight Dark
Matter particles can reproduce the observed positions and brightness of multiply- lensed galaxy images, an important achievement that reveals
the crinkly rather than smooth nature of spacetime around galaxies.

'The possibility that Dark Matter does not comprise ultramassive
particles, as has long been advocated by the scientific community,
alleviates other problems in both laboratory experiments and astronomical observations,' explains Dr Lim.

'Laboratory experiments have been singularly unsuccessful at finding
WIMPs, the long-favoured candidate for Dark Matter. Such experiments are
in their final stretch, culminating in the planned DARWIN experiment,
leaving WIMPs with no place to hide if not found (see remark 2).'
Professor Tom BROADHURST, an Ikerbasque Professor at the University
of the Basque Country, a Visiting Professor at HKU, and a co-author
of the paper adds, 'If Dark Matter comprises ultramassive particles,
then according to cosmological simulations, there should be hundreds of satellite galaxies surrounding the Milky Way. However, despite intensive searches, only around fifty have been discovered so far. On the other
hand, if Dark Matter comprises ultralight particles instead, then the
theory of Quantum Mechanics predicts that galaxies below a certain mass
simply cannot form owing to the wave interference of these particles, explaining why we observe a lack of small satellite galaxies around the
Milky Way.' 'Incorporating ultralight rather than ultramassive particles
for Dark Matter resolve several longstanding problems simultaneously in
both particle physics and astrophysics,' said Amruth Alfred, 'We have
reached a point where the existing paradigm of Dark Matter needs to
be reconsidered. Waving goodbye to ultramassive particles, which have
long been heralded as the favoured candidate for Dark Matter, may not
come easily, but the evidence accumulates in favour of Dark Matter
having wave-like properties as possessed by ultralight particles.'
The pioneering work used the supercomputing facilities at HKU, without
which this work would not have been possible.

The co-author Professor George SMOOT added, 'Understanding the nature
of particles that constitute Dark Matter is the first step towards
New Physics.

This work paves the way for future tests of Wave-like Dark Matter in
situations involving gravitational lensing. The James Webb Space Telescope should discover many more gravitationally-lensed systems, allowing us to
make even more exacting tests of the nature of Dark Matter.' Remarks:
1. The Standard Model of Particle Physics is the theory describing three
of the four known fundamental forces (electromagnetic, weak and strong interactions -- excluding gravity) in the universe and classifying all
known elementary particles. Although the Standard Model has met with huge successes, it leaves some phenomena unexplained -- e.g., the existence
of particles that interact with known particles in the Standard Model
only through gravity -- and falls short of being a complete theory of fundamental interactions.

* RELATED_TOPICS
o Space_&_Time
# Dark_Matter # Astrophysics # Astronomy # Sun #
Solar_Flare # Black_Holes # Galaxies # Northern_Lights
* RELATED_TERMS
o Dark_matter o Ultimate_fate_of_the_universe o Dark_energy
o Subatomic_particle o Galaxy o Interstellar_medium o
Spitzer_space_telescope o Big_Bang

========================================================================== Story Source: Materials provided by The_University_of_Hong_Kong. Note:
Content may be edited for style and length.


========================================================================== Related Multimedia:
* Figures_showing_gravitational_lensing_and_space-time_models ========================================================================== Journal Reference:
1. Alfred Amruth, Tom Broadhurst, Jeremy Lim, Masamune Oguri, George F.

Smoot, Jose M. Diego, Enoch Leung, Razieh Emami, Juno Li, Tzihong
Chiueh, Hsi-Yu Schive, Michael C. H. Yeung, Sung Kei Li. Einstein
rings modulated by wavelike dark matter from anomalies in
gravitationally lensed images.

Nature Astronomy, 2023; DOI: 10.1038/s41550-023-01943-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/04/230425111243.htm

--- up 1 year, 8 weeks, 1 day, 10 hours, 50 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)