Title: Strong Progenitor Age-bias in Supernova Cosmology. II. Alignment with DESI BAO and Signs of a Non-Accelerating Universe
Authors: Junhyuk Son, Young-Wook Lee, Chul Chung, Seunghyun Park, Hyejeon Cho
First Author’s Institution: Department of Astronomy & Center for Galaxy Evolution Research, Yonsei University, Seoul 03722, Korea
Status: Published in Monthly Notices of the Royal Astronomical Society
Are we wrong about dark energy?
For several decades now, astronomers have been operating under a standard model of cosmology known as ΛCDM (see our Guide here!). This model has been quite successful at explaining a wide range of observations with only a few parameters, but like all scientific models, it’s not necessarily perfect, and it’s constantly being tested by new data. In the past few years, the Λ part of ΛCDM has started to be brought into question. Essentially, ΛCDM suggests that our universe is dominated by cold dark matter (CDM) and dark energy, which is the name we give to whatever is responsible for accelerating the expansion of the universe. Our best understanding of dark energy takes the form of a cosmological constant (denoted by the Greek letter Λ). In this scenario, dark energy has a constant density over the entire history of the universe. We can describe the dark energy equation of state using the ratio of its pressure to its energy density, w. In a ΛCDM model, w=-1.
But recent evidence suggests that dark energy may not actually be a constant. Early results from the Dark Energy Spectroscopic Instrument (DESI) suggest that observations actually prefer a situation where w changes over time. This is incredibly intriguing, and you can read more about those results here, here, and here. Today’s paper builds on the DESI results by exploring how a potential bias in our observations of the universe’s expansion could lead to even greater deviations from a ΛCDM model.
A Quick Supernova Tangent
To measure the expansion of the universe, we need to know how fast galaxies are moving away from us and how far away they are. The first can be calculated using redshift, looking at how light gets stretched to longer wavelengths as galaxies are carried away by the expansion of the universe. The second is a bit trickier. Our best tools for calculating the distances to other galaxies are standard candles: objects whose intrinsic brightness is constant or easy to calculate. If we know how bright something is supposed to be, and compare it to how much dimmer it appears to be in our sky, we can calculate its distance!
One of our best standard candles for very distant objects are Type Ia Supernovae. These happen when a white dwarf, the leftover core of a star, is in a binary system with a massive star that is losing mass. That mass can be accreted onto the white dwarf, causing it to grow over time. Once you get a white dwarf above a certain mass (1.44 times the mass of the sun), it becomes unstable and explodes in a Type Ia supernova. Because this always happens at the same mass limit, the luminosity of the supernova is very consistent, making it a standard candle.
A Not So Standard Candle
Over recent years, there has been some evidence that the brightness of Type Ia supernovae might not be constant, and in fact can vary with the properties of the progenitor. Today’s work focuses on how the brightness of the supernovae varies due to the age of the galaxy. They point to previous studies that show that supernovae in younger host galaxies are fainter than those in older galaxies. This suggests that there may be a significant bias in how we’re calculating the distance to these galaxies.
Correcting for this bias is easier said than done. We don’t actually know the age of many of the supernova host galaxies used in cosmological studies. Thus, estimates need to be made based on overall trends we’ve observed. In this work, they apply a simple correction to the measured brightness of supernovae that depends on the host galaxy’s redshift.
Cosmological Implications
They apply this correction to Type Ia supernova data and calculate cosmological parameters (like w) by combining it with measurements of the cosmic microwave background and baryon acoustic oscillations from DESI. They then compare the results against different cosmological models, including ΛCDM and a model where w can vary over time, known as the w0 waCDM model.
Figure 1 shows the Hubble Residual (HR) as a function of redshift for supernova data before and after correcting for the age bias. The HR is the difference between the distance calculated from supernova data and the distance that would be expected using Hubble’s law. After the correction, the supernova data deviates from ΛCDM.
This deviation from ΛCDM has big implications for the future (and present) of the universe. If you recall, dark energy is responsible for driving the accelerated expansion of the universe. But if you change its equation of state, then this acceleration changes. Figure 2 shows this effect for a range of models explored in this work, including ΛCDM (black), the w0 waCDM model derived from the DESI results (green), and the best-fit w0 waCDM model derived using the age-bias corrected supernova data in this work (red).
The Future of the Universe
As seen in Figure 2, under a ΛCDM model, the universe’s expansion is currently accelerating and will continue to do so into the future. This leads to a situation where we face a Heat Death: galaxies continue to move apart from each other forever, stars will die, galaxies will run out of fuel to make more, and the universe will go dark. However, the preliminary DESI results (shown in green in Figure 2) suggest that the expansion of the universe will eventually decelerate. The work done in today’s paper that accounts for the age bias in supernovae (shown in red in Figure 2), argues that not only will the expansion decelerate in the future, but that it is in fact already decelerating. This would be a major paradigm shift in our cosmological understanding! This could even lead to the possibility of a “Big Crunch,” where the universe collapses back in on itself.
So is ΛCDM dead? Well, it’s still somewhat up in the air. While we are starting to see statistically significant deviations from this standard cosmological model, it’s still early days. More observations, especially from big surveys like Rubin LSST and Euclid, will help confirm these results. But smoke is starting to rise around ΛCDM, and there’s definitely a possibility of fire.
Astrobite edited by Sowkhya Shanbhog
Featured image credit: Keck Observatory
Very intriguing… A comment about the “A Not So Standard Candle”: Recently, I read a compilation analysis (2024) that compared measurements of Ho using cosmic distance ladder estimations versus direct estimation methods. There is a discrepancy between these methods, and (removing two very systematic-biased values) direct methods line up with Ho estimations from the CMB sound horizon scale. The point is that it won’t be quite odd that we conclude we were making bad distance estimations all along even without knowing it or just missing valuable data from somewhere (new fundamental physics or unknown astrophysics phenomena). This, of course, represents a great and strong bias at the time of constricting cosmological parameters, e.g., the equation of state of dark energy.