Redefining Cosmic Speed
Amsellem Sharpens Tools to Measure Universe’s Growth
Heidi Opdyke
- Associate Dean of Marketing and Communications, MCS
- Email opdyke@andrew.cmu.edu
- Phone 412-268-9982
The Hubble constant is the universe’s speedometer, measuring how fast galaxies are racing away from each other. Carnegie Mellon University’s Ariel Amsellem is recalibrating that cosmic gauge — developing new techniques to refine our understanding of the universe’s expansion.
“I measure the Hubble constant, which is colloquially known as the expansion rate of the universe, using gravitational waves,” said Amsellem, a graduate student in the Department of Physics and part of the McWilliams Center for Cosmology and Astrophysics.
The expansion of the Universe was discovered by Edwin Hubble in 1929, and researchers are still refining the precise value of its proportionality constant H0. As they hone in on its exact value, they get closer to better understanding dark matter and dark energy.
“The next age of cosmology — especially once the discrepancies amongst H0 measurements are resolved — will involve constraining dark matter and dark energy models,” Amsellem said. “We can constrain some models when it comes to dark matter, but there is a lot of debate on how dark energy behaves. Until we have a handle on the value of H0, it’s difficult to make further progress on our understanding of other cosmological parameters.”
Two methods have been used to measure the Hubble constant: measuring light leftover from the Big Bang and measuring how fast astronomical objects are moving away from Earth. These methods have statistical discrepancies, so the value of the Hubble constant is not very precise.
Amsellem works with one of the newer methods to find the value of the Hubble constant: gravitational wave standard sirens. This method — just eight years old — focuses on detecting gravitational waves from colossal cosmic collisions, like merging black holes or neutron stars.
As these waves come from galaxies, they are affected by the local motion of galaxies, leading to what researchers call peculiar velocities.
Amsellem takes the waves from black hole and neutron star mergers and accounts for these peculiar velocities. By measuring how far the waves have traveled and how fast the galaxies are moving away from Earth, Amsellem and other cosmologists hope to more precisely define the value of H0.
Amsellem works with the Dark Energy Spectroscopic Instrument (DESI) collaboration to gather data on gravitational wave standard sirens and peculiar velocities. He uses computational models to take the data and remove the peculiar velocities.
“It is exciting that we get a chance to shape this field and figure out the pros and cons of this new methodology as we work on it,” said Antonella Palmese, assistant professor of astrophysics and cosmology and Amsellem’s advisor.
“Ariel is an extremely organized, clever and efficient researcher,” Palmese said. “He has been extremely successful with his research and has been making important contributions to the DESI collaboration.”
Amsellem will use the fellowship to continue his work collaborating with DESI.
“It’s super exciting because there’s a lot we don’t know,” Amsellem said. “There’s a lot of opportunity, and hopefully it helps us get the big picture of the universe.”
For his efforts, Amsellem earned the Lederman Fellowship. This new fellowship, created by Frank and Daphna Lederman, assists a graduate student in their research and provides partial funding for tuition and a stipend for an academic year. After earning his Ph.D. in physics from the University of Illinois in 1975, Frank Lederman began a distinguished career in industry focused on advancing applied science and technology. He is best known for leading the development of General Electric’s first medical ultrasound system, a breakthrough that significantly advanced the field of medical imaging. Daphna Lederman studied fine art and graphic design in Jerusalem. She is an active volunteer, dedicating her time to desert landscaping, invasive grass management and cactus rescue efforts.
