SUNSET: 
8The largest sample of Type Ia supernovae ever made by a single telescope sheds light on dark energy.
DES collaboration
We have known for nearly 100 years that the universe is expanding. But only at the turn of the 21st century did astronomers discover that the expansion was actually speeding up. Now, a new study suggests that this phenomenon might be weaker than we thought.
What is Dark Energy?
The initial discovery of cosmic acceleration in the late 1990s came from studying a few dozen Type Ia supernovae, or exploding white dwarf stars. These supernovae are standard candles, meaning they explode with a consistent luminosity, which can then be compared to their apparent brightness to measure their distance. Surprisingly, the light from all these ancient explosions was altered, or shifted, in a way that indicated they were moving away from us — and from each other — much faster than expected; and the further out the explosion was to begin with, the faster it was moving.
Exactly why the universe's expansion rate is accelerating is still an open question. One thing we do know is that this would not be happening if the expansion of the universe were only driven by matter. There’s something else going on, and we don’t know what. Scientists call this cosmic question mark dark energy.
Dark energy is not the opposite of regular energy, nor is it dark. It might not even be energy! It is an unidentified, repulsive quality inherent to the vacuum of space, generally thought to behave the same way everywhere, everywhen. In fact, cosmologists have integrated the simplest version of dark energy, called the cosmological constant, or lambda (Λ), into the standard model of Big Bang cosmology. So far, it has worked better than anything else to explain our observations of the universe.
But now, a new study of thousands of Type Ia supernovae in the Dark Energy Survey (DES), posted on the arXiv astronomy preprint server, suggests that dark energy might not be “as constant as the stars.”
“The reason dark energy is so exciting,” says study lead Tamara Davis (University of Queensland, Australia), “is because it might be one of the few observational probes that can point us in the direction of what a true, complete, theory of physics could be.”
The Dark Energy Survey and Its Supernovae
During a five-year survey, astronomers used a special camera mounted on the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory to discover 1,635 Type Ia supernovae from hundreds of different galaxies spread over a huge range of distances. The light from these supernovae is anywhere between 1 billion and 9 billion years old. Using the aforementioned standard-candle technique, the team calculated the universe’s expansion rate — and established the first good constraints on dark energy.
DES collaboration
In the simplest theory, dark energy is the energy inherent in the vacuum of empty space. It has a kind of negative pressure that stretches spacetime, forcing everything away from everything else. It is constant throughout space and time, which is why it’s called the cosmological constant. But there are other theories, which suggest dark energy might be more or less dense throughout the universe.
To describe dark energy, physicists calculate its equation of state, which is written as Pressure = w * density. The value of w determines the nature of dark energy, and different values have different ramifications for the fate of the universe. The simplest scenario is w=−1, which means that dark energy is a cosmological constant as in the standard model. In this case, the universe will continue to expand until everything cools to absolute zero, in a Big Chill. If w were smaller than −1, the density of dark energy would be increasing, and everything could end much more dramatically in a Big Rip.
“We're basically asking the simple question,” Davis says: “Is dark energy constant? Or does it change?”
By comparing the observed brightness of this massive new dataset of supernovae with theoretical expectations for different values of w, the DES team found a measurement of w = -0.8, meaning the cosmological constant isn't the best fit to their data.
But Davis notes that their result doesn’t create a huge discrepancy. It doesn’t completely rule out the Big Chill, for example, because random fluctuations in the data could reproduce the signal they find about 5% of the time.
“But it's tantalizing enough,” she adds. “Maybe the universe isn't quite as simple as we had thought.”
So, while it’s still possible that dark energy has a constant effect throughout time and space, alternative ideas, with slight variances in the densities of dark energy over cosmic time, are possible. And if you were worried about the Big Rip, you can relax; the new measurement seems to have ruled it out.
A Way Forward
In the meantime, more large-scale observations are needed, to see if the DES results are reproducible.
Whether dark energy turns out to be a cosmological constant or not, the advancement in technology and techniques that the DES represents, and the subsequently enormous contribution of supernovae data the study has provided cannot be understated.
Mickaël Rigault (French National Centre for Scientific Research), who was not a part of the collaboration, is particularly impressed by the machine-learning technique that enabled the group to discover supernovae using only brightness data. “They just used a camera, clipped on a telescope and, with this new technique, discovered not just a few hundred, but more than 1,000 supernovae,” Rigault says.
As for the discrepancy in the dark energy measurement, he’s not so sure. “It’s an interesting fluctuation,” he notes. “I think we can still consider dark energy to be a cosmological constant. But we shall see.”
Comments
Fire-Starter James
January 25, 2024 at 9:01 am
V(r) = c × tanh(Hr/c) predicts the appearance of accelerating expansion.
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Andrew James
January 25, 2024 at 5:23 pm
Untrue. The modified Friedmann equation actually does. e.g.
H^2 = (8πG/3) ρ - (kc^2 / a^2)
where G is the gravitational constant, c is the speed of light, and a is the scale factor that characterizes the size of the universe. Here the energy density (ρ) and the equation of state parameter (w) determine H. The term kc^2 / a^2 represents the curvature of the universe (k = 0 for a flat universe). Dark energy and its effect is additional + Λc^2 in the equation - Λ is the cosmological constant.
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Martian-Bachelor
January 26, 2024 at 10:55 pm
The original result 25 years ago was always somewhat suspect, because these supposed "standard candles" were basically 1/4 magnitude too faint. That's all.
The picture at the top of the article illustrates the problem: how do you measure the brightness of a SN against the surrounding background light of the galaxy it's in? How do you know how much internal absorption there is in the galaxy before the light from the SN leaves it on its way to us?
The vertical extent of the yellow band in the SN Hubble Diagram is greater than the difference between the best fit line in green and the dashed blue line.
Yes, I know the people doing this make reasonable assumptions all along the way and do the best they can, but it's not as simple or certain as it's often made out to be. They could be measuring the brightness evolution of Type Ia SN over time, since I don't think this possibility can be ruled out entirely.
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Andrew James
January 27, 2024 at 12:25 am
Umm. Science works on the principle of finding a number of observations and trying to understand the principle of why. You form a hypothesis, and then further test that hypothesis with either new observations, or getting the observational data that you have, then again re-examine it. In reality, a scientist has to admit that there is always a possibility that the adopted theory may be ruled out entirely. That's incredibly rare.
However our cosmological knowledge is not just based on supernova, as portrayed in this story. It is just another piece in the jigsaw and not the whole sole basis of our understanding. Eliminating and refining observational basis for any theory, is applying different techniques or ideas to come to a reasonable basis of a conclusion.
Saying that the: "The original result 25 years ago was always somewhat suspect, because these supposed "standard candles" were basically 1/4 magnitude too faint. That's all." Wrongly assumes that other cosmologists in the past have not taken into consideration what happens with the light from supernova as it travels towards the observer.
You are also assuming that the maximum light determines the observations here, when it is also true that the shape of the decaying light curve as the supernova expands also tells us of the type and total luminosity of the event. e.g. It's very rare that we know the exact time of most SN maximum. Understanding the true mechanism that causes Type Ia supernova shows that their firepower are very similar. Hence, if we observe these type of SN near to us, we can extrapolate this to more distant ones. This result is independent of the factors of absorption of the light between us and the Type I SN.
After reading this article and the research being done here, my main question is there another way of testing this? This is the force that drives us to understand what's going on, and that's far more exciting than expressing doubt. If we're going to talk about science and cosmology we must always express positivity in the research conducted. Just stating doubt makes novices confused and feeling mislead by those with, for whatever reason, might have other unstated agendas.
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TorbjornLarsson
January 27, 2024 at 6:39 am
"The original result 25 years ago was always somewhat suspect, ..." while in reality it made the matter dominated universe suspect since it resulted in the first self-consistent (LCDM) cosmology that fitted all data. If you look at uncertainties, early "big bang" cosmologies had stars twice the age of the universe and it was still a problem until the discovery of dark energy.
These supernova observations are already better than the earlier ones, even if they rely on photometry still. Planck's Efstathiou has a conference video where he points to the discrepancies in the cosmic ladder - if you accept Planck, BAO et cetera - that lies beyond the Cepheid rung. (IIRC the pivot scale that makes sense of it all is z > 0.005.) JWST has so far been looking at Cepheid/supernova redshifts only half that, but we can expect to see either Eftstahiou or Riess correct (on that point) soon.
While I mainly agree with Andrew James, I note that AFAIK it is merely 70 % of Type 1a supernovas that have "similar ... firepower" since typically [? not an expert, but saw this critique in peer reviewed paper] 30 % of them are discarded. While e.g. real local galaxy and gravitational et cetera results agree with early universe rates. I see no reason to trust supernova results as of yet, or to agree on a Hubble rate. C.f. how early light speed results were inconsistent but agreed on a significantly higher light speed than today's result. It was merely after the methods had been well understood that they gave robust results.
What I do think everyone starts to agree on is a flat LCDM universe. That goes well with other cosmology (slow roll inflation prefers a multiverse, with ours a Goldilocks dark energy density precisely as Weinberg earlier predicted) and high energy physics (the Higgs sector prefers a multiverse, that Goldilocks the masses vs quasistability). Dark matter coupling to gravity is, if simple such as sterile neutrinos, also a Goldilocks mass vs galaxy lifetime candidate. The first and last are well tested respectively testable predictions, the middle is a consistent postdiction. So I have high hopes for modern cosmology, we can move on from criticizing old observations such as dark energy and study the larger questions. A flat universe accepts that both general relativity and quantum field theory are effective. What's not to like!? (Apart from having space and time independent of the fields, same as the fields at inflation vacuum energies would appear independent of each other.)
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Andrew James
January 27, 2024 at 5:02 pm
"...I note that AFAIK it is merely 70 % of Type 1a supernovas that have "similar ... firepower" since typically [? not an expert, but saw this critique in peer reviewed paper] 30 % of them are discarded. While e.g. real local galaxy and gravitational et cetera results agree with early universe rates."
Let's explain for just clarity.
Type Ia supernovae are known as "standard candles" because they have a remarkably consistent luminosity at maximum brightness. Peak absolute magnitude (brightness) of Type Ia supernovae is estimated to be around -19.3±0.3 magnitudes. Similarity of luminosity makes Type Ia supernovae as cosmic distance indicators, enabling them to measure distances to very distant objects.
Most Type Ia supernovae occur in binary star systems consisting of a white dwarf and a companion star, often with another star or a red giant. The mechanism leading to a Type Ia supernova is called the accretion scenario.
where the white dwarf accretes matter from its companion star, slowly gaining mass over time. As the white dwarf's mass approaches a critical limit known as the Chandrasekhar limit c.1.4(2) Solar masses, a runaway nuclear fusion reaction is triggered at 6.02(3)×10^21 pascals. This reaction rapidly consumes carbon and oxygen in the white dwarf, causing a thermonuclear explosion. Errors for SN Ia's are primarily due to the intrinsic variations in their explosion energies. [However, the exactness of collapsing pressure at the atomic level is far more accurately known.) There are residual variations in peak brightness that introduce uncertainties. These variations can be attributed to a range of factors such as the progenitor systems, the explosion mechanisms, and interactions of the SN with its surrounding environment.
Moreover, observational factors can further contribute by errors when measuring luminosity. i.e. The accuracy of distance measurements to Type Ia supernovae impacts the determination of their absolute luminosities. Measurement of their peak magnitudes also relies on the characterization of their light curves, sometime affected by noise and systematic uncertainties.
Reject SN occur because little information has been obtained to deduce when maximum brightness happened or the phase in the diminishing light curve for the observed magnitude(s). e.g. Insufficient information but not an error per se!
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Andrew James
January 27, 2024 at 5:33 pm
Martian-Bachelor. I think my second response explains your questions here.
Also saying; "They could be measuring the brightness evolution of Type Ia SN over time, since I don't think this possibility can be ruled out entirely." That isn't plausible, because the balancing point when white dwarf collapses, is immutable. The only way out is the assumption that the strong force changes over time as the universe ages. That is also unlikely. Why? The underlying principles and mathematical framework that govern the strong force, mediated by particles called gluons and as described by QCD, remain consistent and unchanged. So, in that sense, the strong force itself is considered to be immutable within the context of our current understanding and theories of particle physics.
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TorbjornLarsson
January 27, 2024 at 6:08 am
It's a large collaboration:
"“As the universe expands, the volume of the universe is increasing. But the amount of matter is not. It’s a constant of total matter. So if the volume is increasing and the matter is constant, the density will decrease,” explained Dillon Brout of Boston University, who co-led the cosmological analysis.
“As the universe expands, the volume of the universe is increasing. But the amount of matter is not.”
So far, so good. But dark energy isn’t like that — it has constant density over time. “As the universe expands, the density does not decrease. You get a correspondingly larger total amount of dark energy,” Brout said."
[The Verge]
The DES collaboration has earlier presented the best ever evidence of dark energy (from the cosmological equation of state) and now this. We have to go to eBOSS/SDSS surveys to get a stand alone observation of dark energy (and of flatness/equation of state/LCDM), but this result has the best supernova statistic.
It prefers LCDM if you constrain the universe age to agree with the oldest globular cluster ages - an unconstrained fit is wonky and breaks the universe. But if you do that you can also derive a supernova observation based Hubble rate that sits in between earlier such and Planck/BAO results (from their Hubble rate/age parametrization, in combination with data given in a footnote of their cosmological paper).
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