Searching for dark energy with the whole world's supernova dataset

Apr 21, 2010
Two views of one of the six new distant supernovae in the Supernova Cosmology Project's just-released Union2 survey, which among other refinements compares ground-based infrared observations (in this case by Japan's Subaru Telescope on Mauna Kea) with follow-up observations by the Hubble Space Telescope. Credit: SuprimeCam and WFPC2 via SCP

The international Supernova Cosmology Project (SCP), based at the U.S. Department of Energy's Lawrence Berkeley National Laboratory, has announced the Union2 compilation of hundreds of Type Ia supernovae, the largest collection ever of high-quality data from numerous surveys. Analysis of the new compilation significantly narrows the possible values that dark energy might take, but not enough to decide among fundamentally different theories of its nature.

"We've used the world's best-yet dataset of to determine the world's best-yet constraints on ," says Saul Perlmutter, leader of the SCP. "We've tightened in on dark energy out to redshifts of one," when the universe was only about six billion years old, less than half its present age, "but while at lower redshifts the values are perfectly consistent with a , the most important questions remain."

That's because possible values of dark energy from supernovae data become increasingly uncertain at redshifts greater than one-half, the range where dark energy's effects on the expansion of the universe are most apparent as we look farther back in time. Says Perlmutter of the widening error bars at higher redshifts, "Right now, you could drive a truck through them."

As its name implies, the cosmological constant fills space with constant pressure, counteracting the mutual gravitational attraction of all the matter in the universe; it is often identified with the energy of the vacuum. If indeed dark energy turns out to be the cosmological constant, however, even more questions will arise.

"There is a huge discrepancy between the theoretical prediction for and what we measure as dark energy," says Rahman Amanullah, who led SCP's Union2 analysis; Amanullah is presently with the Oskar Klein Center at Stockholm University and was a postdoctoral fellow in Berkeley Lab's Physics Division from 2006 to 2008. "If it turns out in the future that dark energy is consistent with a cosmological constant also at early times of the universe, it will be an enormous challenge to explain this at a fundamental theoretical level."

A major group of competing theories posit a dynamical form of dark energy that varies in time. Choosing among theories means comparing what they predict about the dark energy equation of state, a value written w. While the new analysis has detected no change in w, there is much room for possibly significant differences in w with increasing redshift (written z).

"Most dark-energy theories are not far from the cosmological constant at z less than one," Perlmutter says. "We're looking for deviations in w at high z, but there the values are very poorly constrained."

In their new analysis to be published in the Astrophysical Journal, "Spectra and HST light curves of six Type Ia supernovae at 0.511 < z < 1.12 and the union2 compilation," the supernova cosmology project reports on the addition of several well-measured, very distant supernovae to the union2 compilation. the paper is now available online at here.

Dark energy fills the universe, but what is it?

Dark energy was discovered in the late 1990s by the Supernova Cosmology Project and the competing High-Z Supernova Search Team, both using distant Type Ia supernovae as "standard candles" to measure the expansion history of the universe. To their surprise, both teams found that expansion is not slowing due to gravity but accelerating.

Other methods for measuring the history of cosmic expansion have been developed, including baryon acoustic oscillation and weak gravitational lensing, but supernovae remain the most advanced technique. Indeed, in the years since dark energy was discovered using only a few dozen Type Ia supernovae, many new searches have been mounted with ground-based telescopes and the Hubble Space Telescope; many hundreds of Type Ia's have been discovered; techniques for measuring and comparing them have continually improved.

In 2008 the SCP, led by the work of team member Marek Kowalski of the Humboldt University of Berlin, created a way to cross-correlate and analyze datasets from different surveys made with different instruments, resulting in the SCP's first Union compilation. In 2009 a number of new surveys were added.

The inclusion of six new high-redshift supernovae found by the SCP in 2001, including two with z greater than one, is the first in a series of very high-redshift additions to the Union2 compilation now being announced, and brings the current number of supernovae in the whole compilation to 557.

"Even with the world's premier astronomical observatories, obtaining good quality, time-critical data of supernovae that are beyond a redshift of one is a difficult task," says SCP member Chris Lidman of the Anglo-Australian Observatory near Sydney, a major contributor to the analysis. "It requires close collaboration between astronomers who are spread over several continents and several time zones. Good team work is essential."

Union2 has not only added many new supernovae to the Union compilation but has refined the methods of analysis and in some cases improved the observations. The latest high-z supernovae in Union2 include the most distant supernovae for which ground-based near-infrared observations are available, a valuable opportunity to compare ground-based and Hubble Space Telescope observations of very distant supernovae.

Type Ia supernovae are the best standard candles ever found for measuring cosmic distances because the great majority are so bright and so similar in brightness. Light-curve fitting is the basic method for standardizing what variations in brightness remain: supernova light curves (their rising and falling brightness over time) are compared and uniformly adjusted to yield comparative intrinsic brightness. The light curves of all the hundreds of supernova in the Union2 collection have been consistently reanalyzed.

The upshot of these efforts is improved handling of systematic errors and improved constraints on the value of the dark energy equation of state with increasing redshift, although with greater uncertainty at very high redshifts. When combined with data from cosmic microwave background and baryon oscillation surveys, the "best fit cosmology" remains the so-called Lambda Cold Dark Matter model, or λCDM.

λCDM has become the standard model of our universe, which began with a big bang, underwent a brief period of inflation, and has continued to expand, although at first retarded by the mutual of matter. As matter spread and grew less dense, dark energy overcame gravity, and expansion has been accelerating ever since.

To learn just what dark energy is, however, will first require scientists to capture many more at high redshifts and thoroughly study their light curves and spectra. This can't be done with telescopes on the ground or even by heavily subscribed space telescopes. Learning the nature of what makes up three-quarters of the density of our universe will require a dedicated observatory in space.

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User comments : 16

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vortex0
2.5 / 5 (4) Apr 21, 2010
This is a very interesting area! Those guys are doing a great job!

However I was intrigued by one statement:
"If it turns out in the future that dark energy is consistent with a cosmological constant also at early times of the universe, it will be an enormous challenge to explain this at a fundamental theoretical level."
If I remember correctly an analysis of mine (around 20 years ago) related to the cosmological constant, it did not seem challenging at all. I hope I am missing something: it would be interesting.

Anyway, I intend to have a closer look at this problem (including reading papers that I missed) and maybe publish some results.
Question
4 / 5 (1) Apr 21, 2010
If we observed a supernova with a 1 Z red-shift would the time curve of brightening and fading double in length? For example would a 1 month curve locally end up being a 2 month curve at 1 Z?
eachus
4.5 / 5 (2) Apr 21, 2010
If we observed a supernova with a 1 Z red-shift would the time curve of brightening and fading double in length? For example would a 1 month curve locally end up being a 2 month curve at 1 Z?


That's the way to bet. The nasty problem is that the expected peak intensity of a type Ia supernova is determined from the light curve. At around z of one, the stretching of the light curve due to relativity has to be corrected for. But how do you know the correction to apply? So the real need is for infrared spectra of the most remote detected supernovas to get an independent measure of red shift. (In other words, a good spectrum for a type 1a can be used to separate distance from velocity.) This is where the James Webb Space Telescope will be very effective. It not only has a larger light gathering capacity than the Hubble, but it is being optimized for far infrared astronomy.
brant
3.2 / 5 (6) Apr 21, 2010
"Dark energy was discovered in the late 1990s by the Supernova Cosmology Project and the competing High-Z Supernova Search Team, both using distant Type Ia supernovae as "standard candles" to measure the expansion history of the universe. To their surprise, both teams found that expansion is not slowing due to gravity but accelerating."

They certainly have not fully quantified the physical SN1a models so it follows that dark energy is a figment.
verkle
1.4 / 5 (11) Apr 21, 2010
Brant, dittos to your comments. Dark Energy still means just something that "may be out there" that we use to explain things that we cannot really explain yet. Same for Dark Matter.

I would be most humble to suggest that we do not even understand 0.1% of the universe yet.

kevinrtrs
1 / 5 (5) Apr 22, 2010
It's just me, an ignoramus dribbling a bit:

There's certainly a lot of excellent work being done to find some elusive entity.

Perhaps we should do a re-think on our starting assumptions for the current cosmological model.

1. We need to decouple present observations from attempts to describe origins, i.e. go with what the data is telling us - only.
2. Re-interpret redshift exactly as it stands with what it is currently giving: Quantised galaxy distribution. With a centre about a mega light years from our location.
3. Accept what WMAP & Ddigital sky map data implies: the universe has poles and hence is turning about a centre.
4. Extremely high Quasar redshifts linked to existing galaxies can be attributed to angular movement.
5. The conclusion could well be: Dark energy & dark matter is simply the angular momentum/velocity/acceleration of galaxies/space around the polar centre of the universe.

Use it, don't use it. Your choice.

But: You did read it here first!
PS3
1.6 / 5 (7) Apr 22, 2010
The expansion is probably a illusion of some sort.
theophys
5 / 5 (1) Apr 22, 2010
Dark Energy still means just something that "may be out there" that we use to explain things that we cannot really explain yet. Same for Dark Matter.


I would tend to agree for dark energy, simply because nobody has a clue as to what it is. Dark matter, less so. We have a lot of good theories for dark matter, from dead stars to WIMPS. That puts dark matter further outside the "god did it" sphere of explanations than dark energy.
frajo
1 / 5 (3) Apr 23, 2010
We have a lot of good theories for dark matter, from dead stars to WIMPS.
You are biased if you exclude the possibility that there isn't any DM.
theophys
3.7 / 5 (3) Apr 26, 2010
We have a lot of good theories for dark matter, from dead stars to WIMPS.
You are biased if you exclude the possibility that there isn't any DM.


I excluded the possibility that there is no dark matter because the data says, "hey, there's a bunch of mass here that we can't see." Observation says it's there so, unless very strong evidense to the contrary pops up, the logical conclusion is that it is there.
frajo
1 / 5 (3) Apr 27, 2010
We have a lot of good theories for dark matter, from dead stars to WIMPS.
You are biased if you exclude the possibility that there isn't any DM.


I excluded the possibility that there is no dark matter because the data says, "hey, there's a bunch of mass here that we can't see." Observation says it's there so, unless very strong evidense to the contrary pops up, the logical conclusion is that it is there.
No, that's not logical. And, no, the data don't say "hey, there's a bunch of mass out there". That's just your interpretation of the data. One legit interpretation, but not the only one. If you dismiss the other legit interpretations you must have an undisclosed agenda to do so. Which is not exactly the scientific way. To be able to dismiss alternative interpretations of the data you'll have to come forth with a suitable particle which is acknowledged by particle physics. Until now, you don't have a suitable particle. We are endulging in hypotheses only.
theophys
3.4 / 5 (5) Apr 27, 2010
No, that's not logical. And, no, the data don't say "hey, there's a bunch of mass out there". That's just your interpretation of the data. One legit interpretation, but not the only one. If you dismiss the other legit interpretations you must have an undisclosed agenda to do so. Which is not exactly the scientific way. To be able to dismiss alternative interpretations of the data you'll have to come forth with a suitable particle which is acknowledged by particle physics. Until now, you don't have a suitable particle. We are endulging in hypotheses only.

Astronomer's have looked at galaxies and noticed motions and interactions that imply gravitational forces at work greater than possible for the luminous matter. In other words, there is more mass than is visible in any spectrum of light we have used. It's not about prefered interpretation or weird agendas. That's what we've seen, thus we have made theories to explain it. There is nothing unscientific or illogical about it.
frajo
2 / 5 (4) Apr 28, 2010
You are just repeating your sentences without addressing my arguments. Why?
theophys
4 / 5 (4) Apr 28, 2010
I'm not sure what your argument is. Are you doubting the validity of the data? Are you doubting the validity of the equations that the data sets were plugged into? You mention biases and agendas because I dismiss certain explanations. That's science my friend, not personal perogative. Explanations that don't agree with data and established facts are discarded on the basis of being blatantly false.
frajo
2 / 5 (4) Apr 29, 2010
I admit not to know the reasons for the galactic missing mass or the dark flow phenomenon, nor for the Pioneer and the flyby anomalies. I admit not to know whether the inverse-square gravitation rule holds true for distances smaller than 0.1 mm or larger than 20 AU. Therefore I can't dismiss hypotheses which assume that gravitation might work a little bit different in all those situations.

But you, theophys ("theo" like in the Greek "theos"?), obviously are wise enough to dismiss any hypothesis which tries to explain some or all of the aforementioned phenomena without resorting to some yet-to-find particle. Congrats. :)
theophys
5 / 5 (3) Apr 30, 2010
Yes. Yes I am.