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Will Dark Energy Tear the Universe Apart?

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Cosmology and its crucial observations begins to collapse. The star doesn't col­ lapse much before the remaining mate­ rial ignites, and then the star explodes Learning about the composition of our universe has involved multiple astro­ with a fantastically bright blast. Because nomical surveys. Below are a collection all type Ia supernovae originate from of the types of observations that have white dwarfs of the same mass, they all led to astronomers' current understand­ have a similar luminosity. ing of the universe. Both groups observed the light curves from type Ia supernovae and found that • Gravitational lensing showed that gal­ the more distant supernovae (which are axy clusters must have additional mass from an earlier time) were dimmer than that isn't luminous. A massive object­ expected if the universe was expanding at in this case, a cluster of galaxies - can a constant rate. This means that the dis­ warp space-time. The light from a dis­ tances between those supernovae and the tant object follows this bent space-time, . which allows the observer to see what telescopes that observed them are greater sits behind the galaxy cluster. The lumi­ J than predicted. The universe's expansion nous mass in a galaxy cluster is not ~ has accelerated over time! enough alone to warp space. ~ So, what is the stuff - dubbed "dark ~ energy" - that's accelerating the expan­ • Galaxy structure surveys showed that j sion rate? This question is one of the big­ luminous matter (galaxy clusters) gest facing science today. clumps in the same way as expected I Everything that we directly observe ­ by dark matter simulations. Luminous people, stars, interstellar medium ­ matter follows the lead of dark matter composes only about 4.6 percent of our because of the gravitational force. universe. What about the other 95.4 per­ •Type la supernovae observations indi­ cent? Through supernova observations cated the universe's expansion is accel­ and CMB observations, astrophysicists erating. Higher red shift type la have determined that roughly 23 percent supernovae (those that are further back of the universe is something called dark in time and therefore farther away) are matter and about 72 percent is dark dimmer than expected. Astronomers energy. Dark matter interacts via the concluded the supernovae must be even but the most recent experiment, the Wilkin­ gravitational force but not the electro­ more distant than expected, which means son Microwave Anisotropy Probe (WMAP), magnetic force, meaning scientists know the universe's expansion is increasing. has been crucial in determining many of the the matter exists, but there's no way to universe's parameters. WMAp, which launched observe it directly. Dark energy is even • Cosmic microwave background (eMB) June 2001, has since measured the tempera­ more bizarre. Most of what astronomers observations first showed in the 1%Os that ture of the universe (2.725 kelvin), how old it microwave radiation emanates from all is (13.7 billion years), the percentage of stuff know about dark energy is that it can be directions in space. This observation was in the universe (4.6 percent normal matter, any type of uniform negative pressure evidence for the Big Bang as the universe's 23 percent dark matter, 72 percent dark energy and that it accelerates the uni­ beginning. A number of eMB experiments energy; all within 1 percent), and the geome­ verse's expansion. have narrowed down eMB characteristics, try of the universe (mostly flat). Wh at is dark energy? Even though cosmologists aren't sure The period of a Cepheid variable star is observer. Hubble compared the distances what dark energy is, they have a few related to its luminosity. By observing the (obtained via Cepheid observations) with ideas. Scientists have three possible dark period and the brightness of the Cepheid, how fast the galaxies appear to be moving energy candidates: quintessence, vacuum Hubble could compare the observed away and noticed a direct correlation: energy, and phantom energy. Each would brightness with the intrinsic luminosity The farther away the galaxy, the faster the result in a different ending to our uni­ to determine the distance to the Cepheid. galaxy moves. The universe is expanding! verse. Which scenario occurs depends Knowing only the distance, though, Schmidt and Perlmutter's teams used both on the value of one parameter and isn't enough to conclude the universe is type Ia supernovae - a different sort of whether that parameter changes in time. expanding. Hubble also looked at the standard candle - for their observa­ This parameter, called the equation of spectra of those galaxies and saw that tions. A type Ia supernova originates state, w, is the ratio between pressure and spectral lines were shifted toward the red from a white dwarf star that is part of a energy density (see "The parameter end of the spectrum. This "redshift" binary star system. The white dwarf pulls everything hinges on:' page 37). means the object is moving away from the material from its binary companion, and A positive value of the equation of once the white dwarf reaches a critical state would cause deceleration in the uni­ Liz Kruesi is an associate editor of Astronomy. mass - 1.4 times that of the Sun - it verse as a result of the gravitational force. 36 Astronomy· Februa ry 09 Each dark energy candidate has negative have an equation of state value more idea of the cosmological constant, calling pressure and therefore a negative equa­ negative than - 'h. The value also deter­ it his greatest "blunder:' tion of state parameter. In fact, in order mines how fast the universe expands. While the cosmological constant looks to generate acceleration, the total And there's more: The equation of state promising as a result of its energy denSity amount of "stuff" in the universe must value does not need to remain constant; - an equation of state of -1 closely fits it can vary in time. CMB observations - the problem arises The parameter ..."0 • Cosmologists split the dark energy when physicists calculate how much vac­ everything hinges on ·~.:: candidates by their equation of state val­ uum energy is expected in the universe. ues. Quintessence has a value between - '13 The standard model of particle phYSics The equation of state, W, does not char­ and -1. It is a dynamic field, meaning its predicts 10 120 times more vacuum energy acterize only a dark energy candidate. density could change over time or from than what scientists observe. The following table shows various values one place to another in the universe. An equation of state parameter more for wand what that value represents. Vacuum energy gets its name from its negative than -1 corresponds to phantom Equation of What that role as the energy of "empty" space. Space energy - the third dark energy candi­ state value (wj value represents is filled with a smooth energy density of date. In this scenario, the universe would virtual particles (particle-antiparticle become progressively more dark-energy­ w=Y! Electromagnetic radiation pairs) that pop in and out of existence. dominated, and acceleration therefore w=O Non-relativistic matter Vacuum energy can be represented by would increase dramatically. So what the cosmological constant term in Albert could this phantom energy be? While w=l Relativistic matter Einstein's general theory of relativity vacuum energy comprises virtual par­ w=-l Cosmological constant, A; because both have an equation of state ticles, "phantom energy might be a per­ vacuum energy value of -1 and therefore have constant verse type of particle that relaxes by - - _ . density as the universe expands. Einstein vibrating faster and faster:' says Robert -Y!>w>-l Quintessence; the value of initially coined the term "cosmological Caldwell of Dartmouth College, lead wcan change with time - -.­ constant" to fit into his static universe author of a 2003 Physical Review Letters w<-l Phantom energy model. After Hubble discovered the uni­ article about phantom energy and its verse is expanding, Einstein retracted the implications for the universe's future. www.Astro nomy.com 37 The future of our universe depends on what dark energy is. If dark energy is phantom energy, the universe's accelerated expansion will increase dramatically and lead the universe to a Big Rip. This timeline shows both the universe's past (from observations and computations) and future (if we're is not too much different, but the energy heading for a Big Rip). In this case, the equation of state is -1.1, which places the Big Rip at roughly increase is just a little bigger:' 86 billion years in the future. ''',,,,,,,,,,y. Roen Kelly. afweakulation. by Roo." Caldwell Like all dark energy candidates, phan­ tom energy has not been directly In the cosmological constant scenario, is negative, and, says Kamionkowski, "so observed, and many questions remain the energy density stays constant; in the the energy per co-moving region in the unanswered. But also like the other dark phantom energy scenario, the energy universe actually increases. With a cosmo­ energy possibilities, cosmologists can density increases. Yet one would expect logical constant, the energy increase is just extrapolate to the universe's future and the energy density of dark energy to large enough to keep the energy density infer how each dark energy candidate decrease as the universe expands in the constant, but the energy per co-moving dictates the universe's end. same way a few drops of colored dye volume is still increasing. Phantom energy dilutes in a tub of water. How do modern Our ultimate fate cosmological theories argue the energy The poet Robert Frost wrote, "Some say density stays constant or even increases? GLOSSARY OF TERMS the world will end in fire / Some say in Marc Kamionkowski of the California ice:' But the universe might hold in store Observable universe Institute of Technology explains, a more violent end.
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