Centauri Dreams
Imagining and Planning Interstellar Exploration
An Incoming ‘Stream’ from Alpha Centauri?
Here’s an interesting thought. We know that at least two objects from outside our Solar System have appeared in our skies, the comet 2I/Borisov and the still enigmatic object called ‘Oumuamua. Most attention on these visitors has focused on their composition and the prospects of one day visiting such an interloper, for it is assumed that with new technologies like the Vera Rubin Observatory and its Legacy Survey of Space and Time (LSST), we will be picking up more of the same.
But consider origins. Extrapolating backward to figure out where either object came from quickly exhausts the most patient researcher, for it only takes the slightest changes in trajectory to widen the search field so broadly as to be useless. That’s especially true since we don’t know the ages of the objects, which may span hundreds of millions of years.
Enter Cole Gregg (University of Western Ontario), who has embarked on a project to study the question from a different perspective. Gregg asks how likely it is that material from another star could be passing through Sol’s neighborhood. As presented at the American Astronomical Society’s Division for Planetary Sciences meeting in Boise, Idaho earlier this month, his calculations explore the gravitational interactions such objects would experience. And he studies whether ‘streams’ of such material could identify by their common characteristics the likely system of origin.
The question is intriguing given how difficult it is to get a probe to Alpha Centauri at present levels of technology. Material from that system, if identified as such, would have interesting implications, especially with regard to the dispersal of chemical elements and organic molecules. The idea of panspermia rides on the possibility that life itself might move between planets and even stars in such material, so getting our hands on something from another star would offer obvious benefits.
Gregg is working on this project with colleague Paul Wiegert, who is perhaps best known for pioneering work with Matt Holman back in the 1990s on planetary orbits possible in the Alpha Centauri system. That work established that rocky terrestrial worlds could remain on stable orbits around Centauri A or B within 3 AU of the host stars, and a bit further out if the planetary orbit was retrograde. As these orbits were stable for billions of years, this finding was an incentive for deeper investigation into Alpha Centauri as a possible home for life.
Gregg and Wiegert now turn their attention to what they call ‘interstellar meteor streams,’ analyzing the development of such streams as they form and “as the material evolves in time through a time-independent, asymmetric potential model for our galaxy.” Their method is to conduct simulations with existing galactic models that involve ejected material in the galactic bulge, the halo and the disk, with the direction of ejection oriented randomly and at speeds of up to 15 kilometers per second.
We’re early in the process here (remember that this work has just been presented), but quoting from a brief write-up made available for a 2023 conference:
As expected, since the physics of their motion is identical, we see the ejecta travel through the disk of the Galaxy in much the same way that stars do. However, through the progression of the simulation we see complicated and interesting effects on the ejecta swarm, hinting at the complicated evolution of interstellar meteoroid streams. This talk will also discuss the implications of the modeled motion on the identification of interstellar meteoroid streams.
For stars in the galactic disk (which is where we are), material ejected from their systems evolves in a way similar to meteoroid streams we see in the Solar System, undergoing ‘orbital shear’. These streams can be long-lived and may include a flux of material passing through our Solar System that, while small, would be of great interest. Gregg sees his ongoing work as refining the process of interstellar transfer and producing population estimates for interstellar visitors currently nearby.
Image: A hypothetical interstellar meteoroid stream after 185 Myr of evolution from a burst ejection originating in the Alpha Centauri system. Credit: Cole Gregg.
As to the Centauri system, Gregg told Sky & Telescope in an article published this month that his calculations using a simplified model of the Milky Way show that about 0.03 percent of material ejected from Alpha Centauri could reach the Solar System, and perhaps most important, be recognized as coming from that source. We’re talking about material that could be as small as dust grains and as large as a comet or asteroid, but moving within a stream that has similar orbital velocities within the galaxy and similar positions, the latter traits making their source identifiable.
Gregg’s work is intriguing and also preliminary, for he intends to produce a full follow-up study looking to refine the galactic model and include better estimates on the likely size of the material that would have made such a crossing. So at this point what we have is only a possibility that may be excluded if the further analysis rules it out. But it’s a worthwhile study given the implications of finding such a stream.
The Sky & Telescope article is from October 18, 2024, titled “Are Objects from Alpha Centauri Streaming by Earth?” (available here). Gregg’s writeup for a 2023 conference is “The Development of Interstellar Meteoroid Streams” (full text). A PowerPoint presentation can be found here. The Wiegert and Holman paper, a key paper in Alpha Centauri studies, is “The Stability of Planets in the Alpha Centauri System,” Astronomical Journal 113 (1997), 1445–1450 (abstract).
SETI: Learning from TRAPPIST-1
Given our decades-long lack of success in finding hard evidence for an extraterrestrial civilization, it hardly comes as a surprise that a recent campaign studying the seven-planet TRAPPIST-1 system came up without a detection. 28 hours of scanning with the Allen Telescope Array by scientists at the SETI Institute and Penn State University produced about 11,000 candidate signals for further analysis, subsequently narrowed down to 2,264 of higher interest. None proved to be evidence for non-human intelligence, but the campaign is interesting in its own right. Let’s dig into it.
The unique configuration of the TRAPPIST-1 planets allowed the scientists involved to use planet-planet occultations (PPOs). A cool M-dwarf star, TRAPPIST-1 brings with it the features that make such stars optimal for detecting exoplanets. The relative mass and size of the planets and star mean that if we’re looking for rocky terrestrial-class worlds, we’re more likely to find and characterize them than around other kinds of star. True, they’re also orbiting a class of star that is dim, but another beauty of TRAPPIST-1 is that it’s only 40 light years out, and we see its seven planets virtually edge-on.
Planets e, f and g can be squeezed into the star’s habitable zone (liquid water on the surface) if we tweak our numbers for possible atmospheres. The edge-on vantage means that planets can pass in front of each other from our viewpoint, with the additional advantage that this well-studied system has planetary orbits that are sharply defined. This raises intriguing possibilities when you consider our own space activities. The Deep Space Network sends powerful signals to communicate with distant craft like the Voyagers, signals wide enough to propagate beyond them and into deep space. The right kind of receiver, if by chance aligned with them, might make a detection, producing evidence for a technology by the nature of its signal.
At TRAPPIST-1, there are seven planet-planet occultations, with two of them involving a potential transmission-source planet within the star’s habitable zone. But we have to consider that transmissions between planets might not be this limited, for radio traffic could move through relays placed for communications purposes on worlds that are uninhabitable. This would obviously be traffic never intended for interstellar reception, the kind of ongoing activity that marks a society communicating with itself, but perhaps leaving a technosignature that would reach the Earth through the width of its beam.
Image: A look from above at the communications line of sight between two worlds in the TRAPPIST-1 system, illustrating the PPO method used in this study. Credit: SETI Institute/Zayna Sheikh.
The possibilities of a detection using this PPO technique vary, of course, with the orbital parameters of the planets in any given system. We must also account for the drift rates produced by orbital motion. The paper explains the recent search’s technique this way:
…it is assumed that the TRAPPIST-1 planets are tidally locked due to their proximity to their host star and will have a negligible rotational contribution to the drift rate of a transmitter on their surfaces. Additionally, their orbital parameters are well constrained, making it possible to calculate the drift rate contributions from their orbital motion. Satellite transmitters in circular orbit around each planet could produce much higher drift rates, up to an additional ∼45 nHz on top of the contribution from the planet’s orbit around the star. However, we have chosen to limit our scope to analogues of our deep space communications, the strongest of which are surface transmitters to deep space probes.
The seven planet-planet occultations studied during the 28 hours of observation ranged from 8.6 minutes to 99.4 minutes. And it turns out that widening that window of observation through simulations produces numerous PPOs with a similarly large range of duration, making this strategy still more interesting. The animation below shows the TRAPPIST-1 system in motion and the possible communications opportunities. Credit: Tusay et al., citation below.
Animation: This is Figure 10 from the paper, the caption of which reads: Simulated potential PPO events during our observations. Online viewers will see a concatenated video of the orbital configuration of the system during each of the observations, including any potential PPO events that we found to occur during those windows. A still image of a PPO event during the observation on Oct 29, 2022 is included where the animation is not accessible. The top panel shows a bird’s-eye view of the system with planet radii scaled up for better viewing. The distances and beam sizes are to scale, assuming a beam created with a 3.4m dish at 3.3 GHz (the maximum frequency observed during this particular session) from the surface of planet g aimed at planet e. The bottom panel shows the edge-on view with planet sizes scaled with distance, showing how much of the beam spills over the planet toward the direction of Earth in the negative z-direction. The red dashed lines in the illustrated beam is the inner angle blocked by the occulting planet, e. The blue dashed lines show the outer angle of the beam that would spill over the planet. The window for this event lasted roughly 95 minutes. Credit: Tusay et al.
Is this method the longest of longshots? SETI itself might be described that way, depending on your views of life in the cosmos. But our steadily growing capabilities at signal detection can’t be ruled out when we consider the possibilities. From the paper:
The analysis of the observations presented here demonstrates that precise characterization of ideal systems, like TRAPPIST-1, enabling orbital dynamical modeling and prediction of PPO events offer practical application for leaked emission searches. This provides SETI a powerful new observational tool and search strategy. As signal detection and RFI mitigation pipelines improve, the inclusion of PPOs to provide narrow search windows may make it more feasible to increase time resolution and sensitivity at higher drift rates.
What beckons most strongly about technosignatures is that they assume no intent (which in any case would be unguessable) on the part of a hypothetical alien civilization. We would essentially be eavesdropping on their activities. Grad student Nick Tusay (Pennsylvania State), lead author of the paper on this work, adds this: “[W]ith better equipment, like the upcoming Square Kilometer Array (SKA), we might soon be able to detect signals from an alien civilization communicating with its spacecraft.” And that would be a SETI detection for the ages.
The paper is Tusay et al., “A Radio Technosignature Search of TRAPPIST-1 with the Allen Telescope Array,” currently available as a preprint.
A Gravitational Wave Surprise
I think gravitational wave astronomy is one of the most exciting breakthroughs we’re tracking on Centauri Dreams. The detection of black hole and neutron star mergers has been a reminder of the tough elasticity of spacetime itself, its interplay with massive objects that are accelerating. Ripples in the fabric of spacetime move outward from events of stupendous energy, which could include neutron star mergers with black holes or other neutron stars. Earth-based observing projects like LIGO (Laser Interferometer Gravitational-Wave Observatory), the European Virgo and KAGRA (Kamioka Gravitational Wave Detector) in Japan continue to track such mergers.
But there is another aspect of gravitational wave work that I’m only now becoming familiar with. It’s background noise. Just as ham radio operators deal with QRN, which is the natural hum and crackle of thunderstorms and solar events, so the gravitational wave astronomer has to filter out what is being called the astrophysical gravitational wave background, or AGWB, as the inevitable acronym would have it. Astronomers also have to consider GW signals associated with events in the early universe, stochastic background ‘static’ that could have originated, for example, in cosmic inflation or the creation of cosmic strings.
The AGWB is the background noise of countless astrophysical events, a ‘hum’ from all sources emitting gravitational waves in the universe. Recent work has been showing that this collective signal, primarily from black hole and binary neutron star mergers, is detectable by the technologies we’ll be deploying in the 2030s in the European Space Agency’s Laser Interferometer Space Antenna (LISA) mission. And it’s clear that for gravitational wave astronomy to proceed, we need to remove the AGWB to uncover underlying signals.
New work now makes the case that, surprisingly, we also have to reckon with the background noise of binary white dwarfs, although I see in the literature that scientists were delving into this as early as 2001 (citation below). In two recent papers, Dutch astronomers have developed models demonstrating that the background noise of white dwarfs would actually be stronger than that produced by black holes. Gijs Nelemans (Radboud University (Nijmegen, the Netherlands), who is working with the software and guidance mechanisms for the LISA mission, is a co-author on two papers on the subject. He sees white dwarf background noise as a way of studying stellar evolution on a galactic scale:
“With telescopes you can only study white dwarfs in our own Milky Way, but with LISA we can listen to white dwarfs from other galaxies. Moreover, in addition to the background noise of black holes and the noise of white dwarfs, perhaps other exotic processes from the early universe can be detected.”
Image: Dutch astronomer Gijs Nelemans. Credit: TechGelderland.
Nelemans has been developing the models described in the two recent papers with students Seppe Staelens and Sophie Hofman. Their work is significant given that until now, the LISA mission had not factored in a noisy white dwarf background problem. In a paper published in Astronomy & Astrophysics, the authors point out:
Given the amplitude of the WD component… it is expected that it can be very well measured by LISA. Furthermore, the relative amplitudes show that, if LISA detects an AGWB signal in the mHz regime, it is likely dominated by the WDs. This means that it is likely hard to make statements about the BH (and NS) population based on a measurement of the AGWB unless there is a way to disentangle the two, or to detect the high-frequency component of the AGWB above 40 mHz.
And in terms of the study of white dwarfs, the paper adds:
This offers an opportunity to study the WD binary population to much larger distances, while hampering the detection of the BH AGWB with missions such as LISA. The WD signal reaches a peak around 10 mHz and at higher frequencies the BH AGWB will become the dominant signal. The detectability of this transition by LISA and other mHz missions ought to be studied in detail.
Image: The LISA mission consists of a constellation of three identical spacecraft, flying in formation. They will orbit the Sun trailing the Earth, forming an equilateral triangle in space. Each side of the triangle will be 2.5 million km long (more than six times the Earth-Moon distance), and the spacecraft will exchange laser beams over this distance. This illustration shows two black holes merging and creating ripples in the fabric of spacetime. Some galaxies are visible in the background. In the foreground, the shape of a triangle is traced by shining red lines. It is meant to represent the position of the three LISA spacecraft and the laser beams that will travel between them. Credit: ESA.
This is indeed a unique kind of probe, because we’re talking about studying white dwarf evolution at high redshift in ways beyond the range of optical astronomy. Realize that only a small selection of gravitational wave sources can be detected with our current technologies. Millions of binaries in the Milky Way will simply merge into the stochastic foreground, a signal that is highly anisotropic (i.e., not uniform in all directions) while unresolved binary sources outside the galaxy produce a background signal that is profoundly isotropic, one that “encodes the combined information about the different source populations,” to quote the Hofman & Nelemans paper.
So we learn that filtering out white dwarf background mergers will be a major part of LISA’s investigations, but that the WD background is also a source of new information. LISA is to be the first dedicated space-based gravitational wave detector, involving three spacecraft in an equilateral triangle 2.5 million kilometers long in a heliocentric orbit. The European Space Agency hopes to launch LISA in 2035 on an Ariane 6.
The papers are Hofman & Nelemans, “On the uncertainty of the white dwarf astrophysical gravitational wave background,” accepted at Astronomy & Astrophysics (preprint); and Staelens & Nelemans, “Likelihood of white dwarf binaries to dominate the astrophysical gravitational wave background in the mHz band,” Astronomy & Astrophysics Vol. 683, A139 (March 2024). Full text. The 2001 paper is “Low-frequency gravitational waves from cosmological compact binaries,” Monthly Notices of the Royal Astronomical Society Vol. 324, Issue 4 (July 2001), pp. 797-810 (abstract).
Catches, Comets and Europa
If the public seems more interested in spaceflight as a vehicle for streaming TV dramas, the reality of both the Europa Clipper liftoff and the astounding ‘catch’ of SpaceX’s Starship booster may kindle a bit more interest in exploring nearby space. When I say ‘nearby,’ bear in mind that on this site the term refers to the entire Solar System, as we routinely discuss technologies that may one day make travel to far more distant targets possible. But to get there, we need public engagement, and who could fail to be thrilled by a returning space booster landing as if in a 1950’s SF movie?
Europa may itself offer another boost if Europa Clipper’s science return is anything like what it promises to be. Closing to 15 kilometers from the surface and making 49 passes over the icy ocean world, the spacecraft may give us further evidence that outer system moons can be venues for life. We also have the European Space Agency’s Jupiter Icy Moons Explorer (JUICE), which will study Europa, Callisto and, in a spectacular move, end up orbiting Ganymede for extended close-up observations.
Image: Europa Clipper begins its journey. Credit: SpaceX.
JUICE gets to Jupiter in July of 2031, while Europa Clipper starts its flybys in the same year, though arriving in 2030. As a measure of how tricky it can be to get to these destinations, both craft make flybys of other worlds, returning in fact to the Earth for some of these. Europa Clipper’s journey will be marked by gravity assists from Mars in February of 2025 and Earth in December 2026. JUICE has already performed one Earth/Moon flyby and will make a flyby of Venus (August, 2025) followed by two Earth flybys (September 2026 and January 2029). A long and winding road indeed!
Speaking of flybys, it’s interesting to note that we have two cometary appearances this month. Comet C/2023 A3 (Tsuchinshan-ATLAS) and C/2024 S1 (ATLAS) are both likely to be visible in October, with the latter closest to Earth on October 24 as it swings toward Sol where it will likely disintegrate. The former should make an appearance in the western sky just after sunset before growing fainter in the latter part of the month. C/2023 A3 appears to be an Oort Cloud object, or long period comet, with an orbital period of some 80,000 years. Short-period comets (Halley’s Comet is one of these) have much shorter orbits, with Halley’s showing up every 76 years.
I find the Oort Cloud a fascinating subject, for it’s based on deduction and not observation. Astronomer James Wray (Georgia Tech), writing in The Conversation, makes the point that while we can’t directly image this vast collection of comets, likely numbering in the hundreds of billions, we can estimate that it extends possibly as far as halfway to the Alpha Centauri system. That’s an intriguing thought, for it means our cometary cloud may intermingle with an equivalent cloud (if one exists) from the Centauri stars. The space covered by our first interstellar probes is not vacant, though the distances between individual objects would still be vast. On the other hand, if the theory that the Oort Cloud formed because of interactions with the giant planets, it’s possible that in the absence of such planets (still not demonstrated), Centauri A and B may not have formed such a cloud.
Wray makes the case that long-period comets are conceivably our greatest planetary threat, outranking near Earth asteroids in degree of danger since an incoming Oort object would likely not be spotted until well inside the planetary system, giving us little time to react. ‘Oumuamua, after all (not an Oort object) was discovered after its closest approach to Earth.
Cometary flybys of our Sun will always be cherished for their visual appeal as ices evaporate and a tail forms, and a collision course with Earth is a highly unlikely scenario, but it’s always best to consider the prospects. Wray puts it this way:
One way to prepare for these objects is to better understand their basic properties, including their size and composition. Toward this end, my colleagues and I work to characterize new long-period comets. The largest known one, Bernardinelli–Bernstein, discovered just three years ago, is roughly 75 miles (120 kilometers) across. Most known comets are much smaller, from one to a few miles, and some smaller ones are too faint for us to see. But newer telescopes are helping. In particular, the Rubin Observatory’s decade-long Legacy Survey of Space and Time, starting up in 2025, may double the list of known Oort Cloud comets, which now stands at about 4,500.
The European Space Agency’s Comet Interceptor mission, scheduled for launch later in this decade, should offer an option for intercepting an Oort Cloud comet when one appears, making it possible to learn more about these objects in terms of their composition and possible role in the delivery of volatiles to the inner system. Oort comets are tricky because their wide orbits mean gravitational influences from other stars can nudge one into a solar close pass without any prior warning. An incoming long-period comet, writes Wray, might offer mere weeks or days to prepare any defense measures we have in place. Even so, the odds of an impact are extremely low.
Image: A stunning return. The Starship booster comes home. Credit: SpaceX.
All this is by way of hoping public interest in space will be quickened both by recent mission successes, ongoing exploration of possible sources of life, and the appearance of the occasional comet. The startling SpaceX success with Starship’s ‘catch’ underlines that technological advances, like comets, can seem to come out of nowhere when we’re not paying attention. I’m thinking back to the science fiction I read as a kid and realizing that watching Starship’s booster descend was right out of Astounding Stories. Heinlein would have loved it, and indeed foreshadowed what unfolded on Sunday.
As SpaceX communications manager Dan Huot put it: “What we just saw, that looked like magic.”
Go Clipper
Is this not a beautiful sight? Europa Clipper sits atop a Falcon Heavy awaiting liftoff at launch complex 39A at Kennedy Space Center. Launch is set for 1206 EDT (1606 UTC) October 14. Clipper is the largest spacecraft NASA has ever built for a planetary mission, 30.5 meters tip to tip when its solar arrays are extended. Orbital operations at Jupiter are to begin in April of 2030, with the first of 49 Europa flybys occurring the following year. The closest flyby will take the spacecraft to within 25 kilometers of the surface. Go Europa Clipper!
Photo Credit: NASA.
In less than 24 hours, NASA's @EuropaClipper spacecraft is slated to launch from @NASAKennedy in Florida aboard a @SpaceX Falcon Heavy rocket.
Tune in at 2pm PT / 5pm ET as experts discuss the prelaunch status of the mission. https://t.co/Nq36BeKieX
— NASA JPL (@NASAJPL) October 13, 2024
Is Dark Energy Truly a Constant?
In a tantalizing article in The Conversation, Robert Nichol (University of Surrey) offers a look at where new physics might just be emerging in conjunction with the study of dark energy. Nichol is an astronomer and cosmologist deeply experienced in the kind of huge astronomical surveys that help us study mind-boggling questions like how much of the universe is made up of matter, dark matter or dark energy. We’ve assumed we had a pretty good idea of their proportions but a few issues do arise.
One of them seems particularly intriguing. Nichol’s article asks whether dark energy, regarded as a constant, may not actually vary over time. That’s quite a thought. The consensus over a universe made up of normal matter (5 percent), dark matter (25 percent) and dark energy (70 percent) came together early in our century, with dark energy taking the role of the cosmological constant Einstein once considered. Although he came to reject the idea, Einstein would doubtless take great interest in the work of observational cosmologists like Nichol, who keep refining the numbers to reduce errors.
Addendum: I hate typos, and thankfully a reader pointed out that in the penultimate sentence above, I had accidentally typed “with dark matter taking the role of the cosmological constant,” when of course it should be dark energy. Now corrected. Not enough caffeine in play this morning, evidently.
Image: The University of Surrey’s Nichol. Credit: University of Portsmouth.
At the heart of the investigation is the Dark Energy Survey, an international effort involving some 400 scientists in seven countries. The survey’s latest numbers, Nichol reports, are that 31.5 percent of the universe is either dark or normal matter, with an error on the measurement of a scant 3 percent. The question of how almost 70 percent of the universe could be in the form of something we can’t see, and something that is indeed not associated in any way with matter, is what Nichol calls “the biggest challenge to physics, even after 20 years of intense study.”
Remember that we first learned of the acceleration of the universe by studying Type Ia supernova (SNeIa) explosions. These occur in binary systems when a white dwarf star begins drawing off material from its companion, usually a red giant. Reaching the Chandrasekhar limit (approximately 1.4 times the mass of the Sun), the white dwarf releases vast amounts of energy, forming a ‘standard candle’ for cosmologists because the luminosity of these events is completely predictable. In other words, supernovae like these have an intrinsic brightness that can be compared to what is observed, making their distance measurable. Plug in the observed redshift and astronomers can use supernovae to make measurements on the rate of the universe’s expansion.
Image: The Hubble Ultra Deep Field, a view of nearly 10,000 galaxies, a reminder of the stunning scope of cosmological studies. The snapshot includes galaxies of various ages, sizes, shapes, and colours. The smallest, reddest galaxies, about 100, may be among the most distant known, existing when the universe was just 800 million years old. The nearest galaxies – the larger, brighter, well-defined spirals and ellipticals – thrived about 1 billion years ago, when the cosmos was 13 billion years old. This image required 800 exposures taken over the course of 400 Hubble orbits around Earth. The total amount of exposure time was 11.3 days, taken between Sept. 24, 2003 and Jan. 16, 2004. Credit: NASA, ESA, and S. Beckwith (STScI) and the HUDF Team.
The Dark Energy Survey has now reported results on such supernovae over a decade of study which included thousands of such events. The paper makes for fascinating reading. Titled “The Dark Energy Survey: Cosmology Results With ∼1500 New High-redshift Type Ia Supernovae Using The Full 5-year Dataset” (citation below), it significantly adds to the number of observed supernovae. There is just a hint here of flexibility in the direction of a variable dark energy. Let me quote the paper:
The standard Flat-ΛCDM cosmological model is a good fit to our data. When fitting DES-SN5YR alone and allowing for a time-varying dark energy we do see a slight preference for a dark energy equation of state that becomes greater (closer to zero) with time (wa < 0) but this is only at the ∼ 2σ level, and Bayesian Evidence ratios do not strongly prefer the Flat-w0waCDM cosmology.
Untangling: The standard Flat-ΛCDM model is the current description of cosmological structure and evolution, using cold dark matter (CDM) and a cosmological constant (Λ). “Flat’ means that the total energy density of the universe equals the critical density (i.e., a flat universe that continues to expand but at ever slower rates). Again, the cosmological constant is what we associate with dark energy and use to explain the accelerating expansion of the universe. And as the paper makes clear, the DES data fit the existing model, but it’s interesting that a dark energy that varies with time is not ruled out, even if the evidence for this is only enough to hint at the possibility.
Now it gets more intriguing. Nichols points out that a second probe looking at Baryon Acoustic Oscillations (BAO), which are “relics of predictable sound waves in the plasma…of the early universe, before the CMB [cosmic microwave background],” likewise hints at the possibility of dark energy that varies with time. This work is being done with the Dark Energy Spectroscopic Instrument (DESI), which has taken position as the successor to the Sloan Digital Sky Survey (SDSS), which had focused on measuring galactic redshifts.
The DESI results are indeed provocative, especially when seen in light of the supernovae results. From the paper on that work (citation below):
…combining any two of the DESI BAO, CMB or SN data sets shows some level of departure from the ΛCDM model. Relaxing the assumption of a spatially flat geometry through varying ΩK [the curvature density parameter] marginally increases the uncertainties but does not change the overall picture. It remains important to thoroughly examine unaccounted-for sources of systematic uncertainties or inconsistencies between the different datasets that might be contributing to these results. Nevertheless, these findings provide a tantalizing suggestion of deviations from the standard cosmological model that motivate continued study and highlight the potential of DESI and other Stage-IV surveys to pin down the nature of dark energy. (italics mine)
As Nichol puts it in his article:
In particular, when DESI analyses the combination of its BAO results with the final DES SNeIa data, the significance of a time-varying dark energy increases to 3.9 sigma (a measure of how unusual a set of data is if a hypothesis is true) – only 0.6% chance of being a statistical fluke.
Most of us would take such odds, but scientists have been hurt before by systematic errors within their data that can mimic such statistical certainty. Particle physicists therefore demand a discovery standard of 5 sigma for any claims of new physics – or less than a one in a million chance of being wrong!
As scientists will say: “Extraordinary claims require extraordinary evidence.”
Indeed. If we do learn that dark energy varies over time, that would mean that there is less of it now than in the past. We would also need to reconsider our notions about the ultimate fate of the universe depending on this new variable. What a time for physics, when the European Southern Observatory is getting ready to start another massive redshift survey and the European Space Agency’s Euclid mission, launched in 2023, is engaged in its own compilation of redshift data. And then there’s the Vera Rubin Observatory in Chile, which will one day soon be adding its own results to the mix. And then there is the quantum question. Adds Nichol:
According to quantum mechanics, empty space isn’t really empty, with particles popping in and out of existence creating something we call “vacuum energy”. Ironically, predictions of this vacuum energy do not agree with our cosmological observations by many orders of magnitude.
So we’re likely to be learning a great deal more in short order, for the Dark Energy Survey continues to compile its own data, and combining these with the above sources should give us a pretty good handle on the question of a variable dark energy. It’s intriguing to think that we may pin down why current dark energy studies are at variance with quantum mechanics. This is new physics of the kind that should make for Nobel Prizes down the road whatever the outcome of the combined data studies. Cosmology is in Nichol’s view likely entering a ‘new era of cosmological discovery.’
The Nichol article is “Dark energy: could the mysterious force seen as constant actually vary over cosmic time?” in The Conversation 10 October 2024 (full text). The DES paper is DES Collaboration, “The Dark Energy Survey: Cosmology Results With ~1500 New High-redshift Type Ia Supernovae Using The Full 5-year Dataset,” Astrophysical Journal Letters Vol. 973, No. 1 (1 October 2024) L14 (full text). The paper on the BAO measurements is DESI Collaboration et al., “DESI 2024 VI: Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations” (abstract) and available in full text as a preprint.
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