SMU research team using spectroscopic data and computer simulation to help measure dark energy

DALLAS (SMU) – SMU researchers working with an international group of astronomers have used early data from the Dark Energy Spectroscopic Instrument (DESI) and a powerful computer simulation to provide precise measurements of the “haloes” of invisible material, termed dark matter, that surround and permeate galaxies.

The team has developed physics calculations related to visible galaxies and quasars to develop accurate models of these hypothetical dark matter regions we cannot see.

A mysterious force known as dark energy is thought to be the cause behind the accelerating expansion of the universe. But since this invisible force -- which is measurable from the distribution of dark matter -- can’t be seen, astronomers will need to do a series of physics calculations about approximately 40 million galaxies and quasars that can be spotted by DESI to better understand dark energy.

“The issue is that the galaxies are not necessarily always where the dark matter is. And sometimes we don’t have a galaxy when we do have dark matter,” said Robert Kehoe, an astrophysics, cosmology and particle physics professor at SMU and co-author of the international study. “What we’ve done is see indirectly, through galaxies, what the dark matter clustering properties are. That will help us fine-tune the data we get from DESI to make the necessary measurements of cosmic expansion.”

He added, “DESI seeks to provide a dramatically improved precision to dark energy measurements, meaning we need to develop enormous and finely detailed simulations to support the measurements.”

The study, providing modelling for bright galaxies, luminous red galaxies, emission line galaxies and quasars was submitted tothe Monthly Notices of the Royal Astronomical Society and a preprint is posted at arXiv. 

Francisco Prada, from the Instituto de Astrofisica de Andalucia (IAA-CSIC), led the study. Other co-authors include Julia F. Ereza, also from the IAA-CSIC; and Alex Smith, from Durham University and The University of Edinburgh.

In addition, James Lasker, a postdoctoral fellow in physics at SMU, and Rajeev Vaisakh, a PhD student in physics at SMU, played a crucial role in the study, having calculated data on two different types of galaxies that cover the most distant portions of the data sample analyzed. Lasker is also a lead operations scientist for DESI and was one of the people who oversaw observations in the telescope at Kitt Peak National Observatory in Arizona. 

Lasker studied the emission line galaxies (ELG) that may cover times when the universe went from decelerating to accelerating expansion. Quasars were studied by Vaisakh, who said “quasars cover the earliest times in our universe, which means they play a key role in understanding the early universe.” 

Unraveling a big question

To study dark energy, which is thought to make up roughly 68 percent of the universe, scientists are using DESI to create an unprecedented 3D map that will provide detailed color spectrum images of millions of galaxies across more than a third of the entire sky. 

Light has been travelling across the universe from the time of the Big Bang, and light emitted from astronomical objects “stretches” and shifts toward the red end of the spectrum over time as the universe expands. Astronomers use that fractional change in the wavelength of light, termed the redshift, to measure how the universe is expanding, and thus to determine the distance to our universe’s most distant – and therefore oldest – objects.

By breaking down the light from each galaxy into its spectrum of colors, DESI can determine how much the light has been redshifted. With this information, physicists are essentially looking to create a measurement tool that tells how far galaxies and other celestial bodies are from the Big Bang creation of the universe billions of years ago, so they can figure out the expansion history of the universe and what role dark energy played at different times. 

But first, one of the things astronomers need to have a better sense of is where these galaxies are in relation to the dark matter halos. 

Astronomers know that dark matter exists around galaxies, creating gravitational pull, because the rules of gravity require sufficient mass in these galaxies to prevent stars from swirling out into space.

Yet, not being able to see dark matter makes it harder for astronomers to precisely model the galaxy-halo connections for different types of galaxies.

“It’s through being sensitive to dark matter clustering that we're able to see the distance scale of the universe over time,” Kehoe explained. 

In this study, the research team did a series of calculations about the galaxy and halo properties for four types of galaxies using two different sources:

• The earliest 1 percent of DESI’s data that has been made available so far for bright galaxies, luminous red galaxies, emission line galaxies and quasars.
• Uchuu, a high-tech computer simulation that relied on the Planck cosmology to estimate where dark matter was in the universe. The analysis team developed models where galaxies would land in this simulation, making a comparison with the DESI 1 percent data possible.

Together, the researchers were able to match the likely locations of dark matter halos in relation to the galaxies spotted by DESI, by estimating important characteristics of the galaxy-halo connection. 

“We produced dozens of independent galaxy catalogs ‘mocking’ or imitating the DESI data catalog based on the Uchuu simulation. These mock galaxy catalogs are created by taking the dark matter halos from the Uchuu simulation and placing galaxies into them in a way that matches the geometry of how they appear to us in the DESI survey,” Lasker said. “From these catalogs, we created a statistical description of how many galaxies, on average, are hosted by haloes of a certain mass.”

This work will provide a good framework for how to model the physics of placing galaxies in dark matter halos when DESI completes its 5-year data collection run, which began in 2021. 

“The goal of DESI is to get down to errors that are sub-percent [for measurements of cosmic evaluations], and that's where the modeling is critical,” Kehoe said. “That is why we must do very careful modeling.” -- Monifa Thomas-Nguyen

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