- 4421 - DARK ENERGY SURVEY - to explain the universe? - By the time DESI has completed its five-year run, it will have mapped over three million quasars and 37 million galaxies. That massive trove of data should help scientists understand if dark energy is changing. Whatever the eventual answer, the question is vital to understanding the Universe.
------------------------- 4421 - DARK ENERGY SURVEY - to explain the universe?
- At the Kitt Peak National Observatory in
Arizona, an instrument with 5,000 tiny robotic eyes scans the night sky. Every
20 minutes, the instrument and the telescope it’s attached to observe a new set
of 5,000 galaxies. The instrument is called DESI—Dark Energy Survey
Instrument—and once it’s completed its five-year mission, it’ll create the
largest 3D map of the Universe ever created.
-
- But scientists are getting access to DESI’s
first data release and it suggests that dark energy may be evolving. DESI is the most powerful multi-object
survey spectrograph in the world. It’s gathering the spectra for tens of
millions of galaxies and quasars. The goal is a 3D map of the Universe that
extends out to 11 billion light-years. That map will help explain how dark
energy has driven the Universe’s expansion.
-
- DESI began in 2021 and is a five-year
mission. The first year of data has been released, and scientists with the
project say that DESI has successfully measured the expansion of the Universe
over the last 11 billion years with extreme precision.
-
- DESI collects light from 5,000 objects at
once with its 5,000 robotic eyes. It observes a new set of 5,000 objects every
20 minutes, which means it observes 100,000 objects—galaxies and quasars—each
night, given the right observing conditions.
-
- We don’t know what dark energy is, but we
know some force is causing the Universe’s expansion to accelerate. DESI measures dark energy by relying on
baryonic acoustic oscillations (BAO.)
Baryonic matter is “normal” matter: atoms and everything made of atoms.
The acoustic oscillations are density fluctuations in normal matter that date
back to the Universe’s beginnings. BAO are the imprint of those fluctuations,
or pressure waves, that moved through the Universe when it was all hot, dense
plasma.
-
- As the Universe cooled and expanded, the
density waves froze their ripples in place, and where density was high,
galaxies eventually formed. The ripple pattern of the BAO is visible in the
DESI leading image. It shows strands of galaxies, or galaxy filaments,
clustered together. They’re separated by voids where density is much lower.
-
- The deeper DESI looks, the fainter the
galaxies are. They don’t provide enough light to detect the BAO. That’s where
quasars come in. Quasars are extremely bright galaxy cores, and the light from
distant quasars creates a shadow of the BAO pattern. As the light travels
through space, it interacts with and gets absorbed by clouds of matter. That
lets astronomers map dense pockets of matter, but it took over 450,000 quasars.
That’s the most quasars ever observed in a survey like this.
-
- Because the BAO pattern is gathered in such
detail and across such vast distances, it can act as a cosmic ruler. By
combining the measurements of nearby galaxies and distant quasars, astronomers
can measure the ripples across different periods of the Universe’s history.
That allows them to see how dark energy has stretched the scale over time.
-
- It’s all aimed at understanding the
expansion of the Universe. In the
Universe’s first three billion years, radiation dominated it. The Cosmic
Microwave Background is evidence of that. For the next several billion years,
matter dominated the Universe. It was still expanding, but the expansion was
slowing because of the gravitational force from matter. But since then, the
expansion has accelerated again, and we give the name dark energy to the force
behind that acceleration.
-
- The “Lambda Cold Dark Matter” (Lambda CDM)
is the standard model of “Big Bang Cosmology”. Lambda CDM includes cold dark
matter—a weakly interacting type of matter—and dark energy. They both shape how
the Universe expands but in opposite ways. Dark energy accelerates the
expansion, and regular matter and dark matter slow it down.
-
- DESI's
five-year run will have mapped over three million quasars and 37 million
galaxies. This should help scientists understand if dark energy is changing. The question is vital to understanding the
Universe.
-
- DESI isn’t the only effort to understand
dark energy. The “ESA’s Euclid spacecraft” is already taking its own
measurements to help cosmologists answer their dark energy questions.
The “Vera Rubin Observatory”
and “Nancy Grace Roman Space Telescope” will both contribute to our
understanding of the elusive dark energy. They’ll perform surveys of their own,
and by combining data from all three, cosmologists are poised to generate some
long-sought answers.
-
-
April 4, 2023 DARK ENERGY
SURVEY - to explain the universe? 4421
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--------------------- --- Sunday, April 7, 2024
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