A new examination of how light bends in the universe suggests a potential alternative theory of gravity.
Recent head-scratching observations of distant galaxies are shaking up cosmologists' established theories about the universe. These findings, from a study in The Astrophysical Journal Letters on June 20, suggest that dark matter, a mysterious substance, might not actually exist. Richard Brent Tully, an astronomer at the University of Hawaii at Manoa not involved in the study, commented that this discovery poses fundamental questions about our understanding of the cosmos.
Astronomers believe in dark matter because stars and visible material at the edge of galaxies rotate differently than expected. Objects far from the galactic center move much faster than they should, based on the visible matter observed through telescopes. According to current physics, this suggests there's a large amount of invisible matter exerting gravitational pull on these stars.
The new findings use the idea that massive objects bend space and time. Galaxies consist of visible stars, gas, dust, and a supposed halo of invisible dark matter. This causes light to bend and distort as it passes near a galaxy, a phenomenon called gravitational lensing.
Astronomer Tobias Mistele and colleagues from Case Western Reserve University in Cleveland used gravitational lensing to study the contents of galaxies. They examined about 130,000 galaxies photographed by the VLT Survey Telescope at the Paranal Observatory in Chile. The team searched for visible streaks that showed distant galaxies whose light had been bent and warped by objects in between.
The level of lensing gives an estimate of the masses of the foreground galaxies, which includes both their visible matter and the presumed larger amount of dark matter. The team then calculated the mass at different distances from the center of each galaxy to determine the orbital speed of stars at those distances.
According to the current cosmological model, Lambda CDM, dark matter forms large clumps in the universe. These clumps exert a gravitational pull, attracting visible matter that forms galaxies. Previous observations show that these halo-like clumps extend outward to at least 300,000 light-years from a galaxy's center. Beyond this distance, the rotation speeds of stars are expected to decrease.
Using their gravitational lensing data, Mistele and his team found that stars placed as far as a million light-years from a galaxy's center, and possibly up to 3 billion light-years away, rotate much faster than expected based on the visible and dark matter assumed to be in the galaxy. This suggests there could be additional invisible material beyond what was previously estimated. However, this conclusion is not definitive.
A competing theory to Lambda CDM, called modified Newtonian dynamics (MOND), rejects the idea of dark matter entirely. Instead, it proposes that gravity behaves differently on the scale of galaxies. Stacy McGaugh, coauthor with Tobias Mistele, supports MOND and suggests it predicts the observations discussed in the study. However, the prevailing Lambda CDM theory, which includes dark matter, still has strong support.
Cosmologist Bhuvnesh Jain from the University of Pennsylvania believes it's premature to dismiss dark matter entirely, as there are many lines of evidence supporting its existence.
Lambda CDM theory provides a better explanation for the growth of large-scale structures in the universe since the Big Bang compared to MOND. Cosmologist Bhuvnesh Jain suggests that more complex mathematical models of gravity, inspired by ideas from string theory involving higher dimensions, could potentially revise our understanding of dark matter's role while explaining the findings from Mistele and his team.
The upcoming data from the European Space Agency's Euclid satellite, launched last year, may offer clearer insights into this intriguing cosmic puzzle.