Dark energy is one of those cosmological features that we are still learning about. While we can\u2019t see it directly, we can most famously observe its effects on the universe – primarily how it is causing the expansion of the universe to speed up. But recently, physicists have begun to question even that narrative, pointing to results that show the expansion isn\u2019t happening at the same rate our math would have predicted. In essence, dark energy might be changing over time, and that would have a huge impact on the universe\u2019s expansion and cosmological physics in general. A new paper available in pre-print on arXiv from Dr. Slava Turyshev, who is also famously the most vocal advocate of the Solar Gravitational Lens mission, explores an alternative possibility that our data is actually just messy from inaccuracies in how we measure particular cosmological features – like supernovae.<\/p>\n
The debate stems from the Dark Energy Spectroscopic Instrument (DESI) releasing its second batch of data, known as DR2 in astronomy jargon. Previous papers had found a disconnect between DESI\u2019s new galaxy maps and the Cosmic Microwave Background that is the leftover remnants of the Big Bang. One potential explanation for that mismatch is that dark energy is \u201cevolving\u201d – either getting stronger or weaker over the course of billions of years.<\/p>\n
Not so fast, says Dr. Turyshev. Extraordinary claims require extraordinary evidence, and there\u2019s one very clear potential error that could explain the disconnect between DESI DR2 and the CMB. If our measurements of supernovae are off, even by 0.02 magnitudes, it could explain the disconnect. Supernovae are commonly used in distance measurements at cosmological scales, so getting their brightness down exactly is critical to correctly measuring distance. And Dr. Turyshev, like many other astrophysicists, isn\u2019t sure our current crop of telescopes is up to that task.<\/p>\n<\/p>\n
Fraser discusses how we know dark energy exists <\/p>\n
Another potential point of error is a \u201ccosmic ruler\u201d that is used in these scenarios. Known as the \u201csound horizon\u201d (which sounds like a great name for a metal band), it measures the distance a clump of matter would move from their starting points out into the universe, but at a very specific speed – the speed of sound in the hot plasma that made up the early universe. These waves, known as Baryon Acoustic Oscillations, lasted for about 380,000 years, before coming to a halt when the universe cooled down enough for the first atoms to form, essentially freezing them in place.<\/p>\n
We use that distance as a ruler to measure distances to other things spread throughout the universe. But, since it is again a measurement, slight errors in the instruments used to calculate that measurement can introduce further errors down the line. To rectify this, Dr. Turyshev suggests a mathematical trick called the Alcock-Paczynski (AP) diagnostic. Instead of using the sound horizon, this technique uses a calculated shape of the universe that isn\u2019t reliant on fuzzy measurements of a certain point in the universe\u2019s early history.<\/p>\n
If dark energy still seems to be fluctuating even after those checks are made, Dr. Turyshev has some potential answers as to why. He came up with a new one, called the Late-Transition Interacting Thawer (LTIT) model, which models how dark energy could \u201cthaw\u201d after a certain amount of time after the universe started and is slowly beginning to interact with dark energy more and more, which is what we see as the expansion of the universe. <\/p>\n<\/p>\n
Fraser interviews Dr. Dillon Brout about how important supernovae are to understanding dark energy.<\/p>\n
Another potential explanation is known as the \u201cPhantom Crossing\u201d, where dark energy might become extremely powerful at some point, transitioning to what is called \u201cphantom\u201d energy. But if this theory is true, according to Dr. Turyshev, we will need a whole new set of physics to explain it as it doesn\u2019t fit the standard model at all. <\/p>\n
Ultimately, we\u2019re still collecting more evidence on dark energy and all its associated mysteries. Luckily, even more data is coming, and some is already here. Euclid, another cosmological probe, recently released its first data set, and astrophysicists are already pouring over it in the hopes of shining some more light on this dark force in the universe. There\u2019s still plenty more discoveries left to come in this space, as DESI is also actively collecting data for its third data release, which will contain data from the first three years of the main survey, hopefully later this year.<\/p>\n
Learn More:<\/p>\n
S. G. Turyshev – Dark Energy After DESI DR2: Observational Status, Reconstructions, and Physical Models<\/a><\/p>\n UT – Could Dark Energy Be Evolving Over Time?<\/a><\/p>\n UT – Fresh Findings Strengthen the Case for Dark Energy’s Evolution<\/a><\/p>\n