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Home , Big Rip, Phantom energy

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 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 ) were dimmer than that isn't luminous. A massive object­ expected if the universe was expanding at in this case, a cluster of - 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 cluster. The lumi­ J than predicted. The universe's expansion nous mass in a 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 (galaxy clusters) gest facing science today. clumps in the same way as expected I Everything that we directly observe ­ by 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. 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 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: , Hubble could compare the observed away and noticed a direct correlation: energy, and . 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 "" 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 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 , 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 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 . 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 Institute of Technology explains, a more violent end. Cosmologists have The region of the universe that "Whether the energy density dilutes or astronomers can observe. It's gov· theorized many ending scenarios to our not depends on the equation of state of erned by the finite speed of light universe, but its ultimate fate will depend the dark energy:' and the universe's age: 13.7 billion on the behavior of dark energy. To further understand this concept, years. As far as we know, beyond If the dark energy density were to dis­ compare a box filled with hot gas to a this limit, the universe could be infi­ appear, matter and radiation ultimately region of the expanding universe that nite. Many use the term "universe" would dominate the energy density of the you observe. (This area is called a "co­ to mean the observable universe. universe. In this scenario - the Big moving" region because you as an Crunch - the attractive gravitational observer are moving with the region force would take over, and the universe's A function of time that represents you're observing.) In the box, Kamion­ contents would collapse into a singularity the relative size of the expanding kowski explains, "the high pressure asso­ - likely a black hole. Given what scien­ universe; the current value is 1. ciated with the heat may push the walls tists know about dark energy (mainly that of the box outward. The heat energy in Hubble length it is accelerating the expansion), this Big the box then decreases, but energy is still The radius of the observable uni­ Crunch scenario is not the most likely conserved. The energy lost from the box verse, which is the region of the uni­ ending. However, one can't rule it out. transfers some energy to the outward verse that astronomers can observe. What if the dark energy density stays motion of the walls. Likewise, in an constant as the universe expands, as expanding universe, the change in the Singularity occurs in the cosmological constant sce­ energy in a co-moving region of the uni­ A point at which space and time are nario? In this situation - the Big Chill ­ verse goes toward pushing the adjacent extremely distorted; where the cur­ the universe's expansion would continue vature of the universe goes to zero regions of the universe outward:' to accelerate, but the acceleration would or infinity. Recall that the dark energy equation not increase. Stars will burn out, galaxies of state is negative; therefore the pressure will pass beyond the Hubble distance,

38 Astronomy· February 09 the universe that is held by the gravitational force will dissoci­ About 1 hour ate. The horizon radius shrinks to a before the end, point, and all matter will rip apart. First, phantom energy the Local Group of galaxies will be ripped apart, followed by the Milky Way will tear Galaxy. As phantom dark energy contin­ than -1, Earth apart. ues to increase, it will rip our planet from then the Big the Sun roughly a year before the end of Rip occurs the universe. About 1 hour before the latd' The key is end, phantom energy will tear Earth to narrow the and space will become empty and cold. apart. But it won't stop there. value of w, the The CMB radiation (the radiation that After all gravitationally bound objects equation of state. permeates space) will cool to just a frac­ are ripped apart, and just fractions of a "Over the past 5 years, tion of a degree above absolute zero. In second before the end of the universe, observations have con­ the Big Chill, the universe doesn't actu­ phantom energy will rip apart all objects strained w to be closer to ally end; it expands forever. held together via electromagnetic and -1;' he says. Ifdark energy is phantom energy, strong forces. These objects include mol­ Remember that an equa­ however, we can count on a far more vio­ ecules, , and even subatomic par­ tion of state value of -1 cor­ lent end to our universe - the Big Rip. ticles. Then the universe will end in a responds to the cosmological singularity, but a different sort of singu­ constant. That's how close The Big Rip larity than the Big Bang and the Big these two scenarios are. A Recall that if the equation of state is less Crunch. In this scenario, instead of all value of -1 implies a Big Chill, than -1, and dark energy is phantom matter and radiation being squashed while a value less than -1 leads energy, the universe would become together, all the universe's components to a Big Rip. "It couid be that the increasingly dark-energy-dominated and would be ripped apart to infinity. true w takes the value of -1.05, the acceleration would increase. When And that is the Big Rip. It sounds which is also consistent with cur­ playing out this scenario, the scale factor great, doesn't it? rent data, and yet it will lead to the - the relative expansion of the universe Fortunately, there's no need to worry Big Rip;' explains Dragan Huterer - blows up to infinity. As the scale factor quite yet. The Big Rip "would not be of the University of Michigan. grows larger than the Hubble distance, before [about] 55 billion years in the The possibility of the Big Rip, and galaxies disappear beyond the horizon of future, if at all;' Caldwell says. Some cal­ when it would occur, teeters on the the observable universe. Similar to the culations say it would occur almost 90 value of one parameter. Now cos­ Big Chill, any observers left on Earth billion years from now. mologists just need to determine that would see fewer galaxies. The big differ­ Kamionkowski, co-author of the 2003 value and whether it holds constant. '!. ence, however, between the Big Chill and article about phantom energy and the Big the Big Rip is what occurs next. Rip, explains: "If w is far less than -1, Learn about the future detectors that may determine dark energy's Phantom energy will strip apart gravi­ then the Big Rip occurs relatively soon. w at www.Astronomy.com/toc. tationally bound objects. Everything in If it is extremely close to -1, but still less