The universe we see is just the very tip of the huge cosmic iceberg.
The lots of of billions of galaxies it incorporates, every of them dwelling to billions of stars, planets and moons in addition to large star-and-planet-forming clouds of fuel and dirt, and the entire seen mild and different power we are able to detect within the type of electromagnetic radiation, corresponding to radio waves, gamma rays and X-rays — briefly, every little thing we’ve ever seen with our telescopes — solely quantities to about 5% of all of the mass and power within the universe.
Together with this so-called regular matter there may be additionally darkish matter, which may’t be seen, however may be noticed by its gravitational impact on regular, seen matter, and makes up one other 27% of the universe. Add them collectively, they usually solely whole 32% of the mass of the universe — so the place’s the opposite 68%?
So what precisely is darkish power? Put merely, it’s a mysterious pressure that’s pushing the universe outward and inflicting it to increase sooner because it ages, engaged in a cosmic tug-of-war with darkish matter, which is attempting to drag the universe collectively. Past that, we don’t but perceive what darkish power is, however Penn State astronomers are on the core of a gaggle that’s aiming to seek out out by means of a novel and bold mission 16 years within the making: HETDEX, the Passion-Eberly Telescope Darkish Vitality Experiment.
“HETDEX has the potential to vary the sport,” mentioned Affiliate Professor of Astronomy and Astrophysics Donghui Jeong.
Darkish power and the increasing universe
In the present day there may be consensus amongst astronomers that the universe we inhabit is increasing, and that its growth is accelerating, however the concept of an increasing universe is lower than a century outdated, and the notion of darkish power (or anything) accelerating that growth has solely been round for slightly greater than 20 years.
In 1917 when Albert Einstein utilized his basic principle of relativity to explain the universe as an entire, laying the foundations for the massive bang principle, he and different main scientists at the moment conceived of the cosmos as static and nonexpanding. However to be able to preserve that universe from collapsing below the engaging pressure of gravity, he wanted to introduce a repulsive pressure to counteract it: the cosmological fixed.
It wasn’t till 1929 when Edwin Hubble found that the universe is in truth increasing, and that galaxies farther from Earth are shifting away sooner than these which can be nearer, that the mannequin of a static universe was lastly deserted. Even Einstein was fast to change his theories, by the early 1930s publishing two new and distinct fashions of the increasing universe, each of them with out the cosmological fixed.
However though astronomers had lastly come to know that the universe was increasing, and had roughly deserted the idea of the cosmological fixed, in addition they presumed that the universe was dominated by matter and that gravity would ultimately trigger its growth to sluggish; the universe would both proceed to increase eternally, however ever-increasingly slowly, or it could sooner or later stop its growth after which collapse, ending in a “huge crunch.”
“That’s the way in which we thought the universe labored, up till 1998,” mentioned Professor of Astronomy and Astrophysics Robin Ciardullo, a founding member of HETDEX.
That yr, two impartial groups — one led by Saul Perlmutter at Lawrence Berkeley Nationwide Laboratory, and the opposite led by Brian Schmidt of the Australian Nationwide College and Adam Riess of the Area Telescope Science Institute — would practically concurrently publish astounding outcomes displaying that the growth of the universe was in truth accelerating, pushed by some mysterious antigravity pressure. Later that yr, cosmologist Michael Turner of the College of Chicago and Fermilab coined the time period “darkish power” to explain this mysterious pressure.
The invention can be named Science journal’s “Breakthrough of the Yr” for 1998, and in 2011 Perlmutter, Schmidt and Reiss can be awarded the Nobel Prize in physics.
Greater than 20 years after the invention of darkish power, astronomers nonetheless don’t know what, precisely, it’s.
“Every time astronomers say ‘darkish,’ meaning we don’t have any clue about it,” Jeong mentioned with a wry grin. “Darkish power is simply one other approach of claiming that we don’t know what’s inflicting this accelerating growth.”
There are, nevertheless, plenty of theories that try to elucidate darkish power, and some main contenders.
Maybe essentially the most favored clarification is the beforehand deserted cosmological fixed, which modern-day physicists describe as vacuum power. “The vacuum in physics is just not a state of nothing,” Jeong defined. “It’s a place the place particles and antiparticles are constantly created and destroyed.” The power produced on this perpetual cycle might exert an outward-pushing pressure on house itself, inflicting its growth, initiated within the huge bang, to speed up.
Sadly, the theoretical calculations of vacuum power don’t match the observations — by an element of as a lot as 10120, or a one adopted by 120 zeroes. “That’s very, very uncommon,” Jeong mentioned, “however that’s the place we’ll be if darkish power seems to be fixed.” Clearly this discrepancy is a significant challenge, and it might necessitate a transforming of present principle, however the cosmological fixed within the type of vacuum power is nonetheless the main candidate to this point.
On account of its design, HETDEX is amassing an enormous quantity of knowledge, extending effectively past its meant targets and offering extra insights into issues like darkish matter and black holes, the formation and evolution of stars and galaxies, and the physics of high-energy cosmic particles corresponding to neutrinos.
One other attainable clarification is a brand new, yet-undiscovered particle or subject that may permeate all of house; however to this point, there’s no proof to assist this.
A 3rd risk is that Einstein’s principle of gravity is inaccurate. “In case you begin from the improper equation,” Jeong mentioned, “you then get the improper reply.” There are alternate options to basic relativity, however every has its personal points and none has but displaced it because the reigning principle. For now, it’s nonetheless the most effective description of gravity we’ve obtained.
In the end, what’s wanted is extra and higher observational knowledge — exactly what HETDEX was designed to gather like no different survey has finished earlier than.
A map of stars and sound
“HETDEX could be very formidable,” Ciardullo mentioned. “It’s going to watch 1,000,000 galaxies to map out the construction of the universe going over two-thirds of the way in which again to the start of time. We’re the one ones going out that far to see the darkish power element of the universe and the way it’s evolving.”
Ciardullo, an observational astronomer who research every little thing from close by stars to faraway galaxies and darkish matter, is HETDEX’s observations supervisor. He’s fast to notice, although, that he’s obtained assist in that function (from Jeong and others) and that he and everybody else on the mission wears a couple of hat. “This can be a very huge mission,” he mentioned. “It’s over $40 million. However for those who rely heads, it’s not very many individuals. And so all of us do a couple of factor.”
Jeong, a theoretical astrophysicist and cosmologist who additionally research gravitational waves, was instrumental in laying the groundwork for the examine and is closely concerned within the mission’s knowledge evaluation — and he’s additionally serving to Ciardullo decide the place to level the 10-meter Passion-Eberly Telescope, the world’s third largest. “It’s sort of fascinating,” he famous with a chuckle, “a theorist telling observers the place to look.”
“We’re the one ones going out that far to see the darkish power element of the universe and the way it’s evolving.”
— Robin Ciardullo, Penn State professor of astronomy and astrophysics
Whereas different research measure the universe’s growth utilizing distant supernovae or a phenomenon often called gravitational lensing, the place mild is bent by the gravity of large objects corresponding to galaxies and black holes, HETDEX is concentrated on sound waves from the massive bang, referred to as baryonic acoustic oscillations. Though we are able to’t truly hear sounds within the vacuum of house, astronomers can see the impact of those primordial sound waves within the distribution of matter all through the universe.
Throughout the first 400,000-or-so years following the massive bang, the universe existed as dense, sizzling plasma — a particle soup of matter and power. Tiny disturbances referred to as quantum fluctuations in that plasma set off sound waves, like ripples from a pebble tossed right into a pond, which helped matter start to clump collectively and kind the universe’s preliminary construction. The results of this clumping is clear within the cosmic microwave background (additionally referred to as the “afterglow” of the massive bang), which is the primary mild, and the farthest again, that we are able to see within the universe. And it’s additionally imprinted within the distribution of galaxies all through the universe’s historical past — just like the ripples on our pond, frozen into house.
“The physics of sound waves is fairly well-known,” Ciardullo mentioned. “You see how far this stuff have gone, you understand how quick the sound waves have traveled, so you realize the space. You’ve got an ordinary ruler on the universe, all through cosmic historical past.”
Because the universe has expanded so has the ruler, and people variances within the ruler will present how the universe’s fee of growth, pushed by darkish power, has modified over time.
“Mainly,” Jeong mentioned, “we make a three-dimensional map of galaxies after which measure it.”
New discovery house
To make their million-galaxy map, the HETDEX staff wanted a robust new instrument.
A set of greater than 150 spectrographs referred to as VIRUS (Seen Integral-Area Replicable Unit Spectrographs), mounted on the Passion-Eberly Telescope, gathers the sunshine from these galaxies into an array of some 35,000 optical fibers after which splits it into its element wavelengths in an ordered continuum often called a spectrum.
Galaxies’ spectra reveal, amongst different issues, the pace at which they’re shifting away from us — a measurement often called “redshift.” Because of the Doppler impact, the wavelength of an object shifting away from its observer is stretched (consider a siren that will get decrease in pitch because it speeds away), and an object shifting towards its observer has its wavelength compressed, like that very same siren rising in pitch because it will get nearer. Within the case of receding galaxies, their mild is stretched and thus shifted towards the pink finish of the spectrum.
Measuring this redshift permits the HETDEX staff to calculate the space to these galaxies and produce a exact three-dimensional map of their positions.
Among the many galaxies HETDEX is observing are what are often called Lyman-alpha galaxies — younger star-forming galaxies that emit sturdy spectral traces at particular ultraviolet wavelengths.
“We’re utilizing Lyman-alpha-emitting galaxies as a ‘tracer particle,’” defined Analysis Professor of Astronomy and Astrophysics Caryl Gronwall, who can also be a founding member of HETDEX. “They’re straightforward to seek out as a result of they’ve a really sturdy emission line, which is simple to seek out spectroscopically with the VIRUS instrument. So we have now this methodology that effectively picks out galaxies at a reasonably excessive redshift, after which we are able to measure the place they’re, measure their properties.”
Gronwall, who together with Ciardullo has been finding out Lyman-alpha galaxies for practically 20 years, leads HETDEX’s efforts on this space, whereas Affiliate Professor of Astronomy and Astrophysics Derek Fox lends his experience to calibrating the VIRUS instrument, utilizing incidental observations of stars with well-known properties to fine-tune its spectra.
“Each shot we take with HETDEX, we observe some stars on the fibers,” Fox defined. “That’s a chance, as a result of the celebs are telling you ways delicate your experiment is. If you realize the brightness of the celebs and also you see the information that you just acquire on them, it presents a chance to maintain your calibration on level.”
“HETDEX has the potential to vary the sport.”
— Donghui Jeong, Penn State affiliate professor of astronomy and astrophysics
Considered one of HETDEX’s greatest strengths is that it was designed as a blind survey — observing broad swaths of sky as an alternative of particular, predetermined objects. “No one has tried doing a survey like this earlier than,” Ciardullo mentioned. “It’s all the time ‘Discover your objects, then do the spectroscopy.’ We’re the primary ones to attempt to do an entire lot of spectroscopy after which work out what we noticed.”
On account of this design, HETDEX is amassing an enormous quantity of knowledge, extending effectively past its meant targets and offering extra insights into issues like darkish matter and black holes, the formation and evolution of stars and galaxies, and the physics of high-energy cosmic particles corresponding to neutrinos.
“That’s very completely different and really fascinating,” Jeong mentioned. “We’ve got big discovery house.”
Ciardullo added, “One factor you possibly can infer — for those who first must see an object earlier than pointing your spectroscope there, effectively that’s nice, nevertheless it requires that the thing be capable to be seen. HETDEX can observe spectra of issues that you may’t see.”
Which means that along with the recognized knowledge it’s amassing, HETDEX is opening a window to surprising findings, discoveries but unexpected. “We might be a pathfinder for extra experiments,” Ciardullo mentioned, and that sentiment is echoed by others on the staff, together with Fox.
“We’re positively going to be blazing trails on the market,” he mentioned. “There’s huge, huge potential for actually thrilling discoveries.”
Again to roots, and past
The futuristic science of HETDEX is, in a wierd twist, very a lot in keeping with the concepts that drove the event of the Passion-Eberly Telescope (HET) practically 40 years in the past.
“HET was initially conceived because the Penn State Spectroscopic Survey Telescope,” defined Professor Emeritus of Astronomy and Astrophysics Larry Ramsey, who invented the telescope in 1983 with then Penn State colleague Dan Weedman, and later served as chairman of the HET’s board of administrators. “The unique mission was to conduct spectroscopic surveys, and within the virtually 20 years between once we first devoted the telescope and once we began HETDEX, the telescope was probably not doing surveys. So in a really actual sense HETDEX is taking the HET again to its roots, and it has grown into a extremely fascinating mission.”
“The size of this survey could be very futuristic, even now,” Jeong mentioned. Recalling a current cosmology convention, he associated a dialogue about the way forward for galactic surveys. “I sat there and listened, and it was mainly what we’re doing,” he mentioned. “HETDEX is a future survey that exists now.”
Along with what HETDEX discovers about darkish power, the information it’s amassing may even present fodder for future research far past the scope of its personal mission. And chances are high, HETDEX will proceed doing “spacebreaking” science on the distant, high-redshift universe for fairly a number of years to return.
“Even at the moment deliberate future surveys don’t transcend HETDEX,” Jeong mentioned. “I feel we’ll nonetheless be on the forefront, even 10 years from now.”