IEEE Spectrum October, 2012 - 43

cosmic web: the Bolshoi simulation
models the evolution of dark matter,
which is responsible for the largescale structure of the universe. Here,
snapshots from the simulation
show the dark matter distribution at
500 million and 2.2 billion years [top]
and 6 billion and 13.7 billion years
[bottom] after the big bang. these
images are 50-million-light-year-thick
slices of a cube of simulated universe
that today would measure roughly
1 billion light-years on a side and
encompass about 100 galaxy clusters.
SourceS: Simulation, anatoly Klypin and Joel r. primacK;
viSualization, Stefan Gottlöber/leibniz inStitute for
aStrophySicS potSdam

To understand the cosmos,
we must evolve it all over again
By Joel r. primack

W

hen it comes to reconstructing the past,
you might think that astrophysicists have it easy. after all,
the sky is awash with evidence. For most of the universe's
history, space has been largely transparent, so much so
that light emitted by distant galaxies can travel for billions of years before
finally reaching earth. it might seem that all researchers have to do to
find out what the universe looked like, say, 10 billion years ago is to build
a telescope sensitive enough to pick up that ancient light.
actually, it's more complicated than that. most of the ordinary matter
in the universe-the stuff that makes up all the atoms, stars, and galaxies
astronomers can see-is invisible, either sprinkled throughout intergalactic
space in tenuous forms that emit and absorb little light or else swaddled
inside galaxies in murky clouds of dust and gas. When astronomers look
out into the night sky with their most powerful telescopes, they can see no
more than about 10 percent of the ordinary matter that's out there.
to make matters worse, cosmologists have discovered that if you add
up all the mass and energy in the universe, only a small fraction is composed of ordinary matter. a good 95 percent of the cosmos is made up of two
very different kinds of invisible and as-yet-unidentified stuff that is "dark,"
meaning that it emits and absorbs no light at all. one of these mysterious
components, called dark matter, seems immune to all fundamental forces
except gravity and perhaps the weak interaction, which is responsible for

www.spectrum.ieee.org

octobEr 2012 * iEEE SpEctrum * NA

43



http://www.spectrum.ieee.org

Table of Contents for the Digital Edition of IEEE Spectrum October, 2012

IEEE Spectrum October, 2012 - Cover1
IEEE Spectrum October, 2012 - Cover2
IEEE Spectrum October, 2012 - 1
IEEE Spectrum October, 2012 - 2
IEEE Spectrum October, 2012 - 3
IEEE Spectrum October, 2012 - 4
IEEE Spectrum October, 2012 - 5
IEEE Spectrum October, 2012 - 6
IEEE Spectrum October, 2012 - 7
IEEE Spectrum October, 2012 - 8
IEEE Spectrum October, 2012 - 9
IEEE Spectrum October, 2012 - 10
IEEE Spectrum October, 2012 - 11
IEEE Spectrum October, 2012 - 12
IEEE Spectrum October, 2012 - 13
IEEE Spectrum October, 2012 - 14
IEEE Spectrum October, 2012 - 15
IEEE Spectrum October, 2012 - 16
IEEE Spectrum October, 2012 - 17
IEEE Spectrum October, 2012 - 18
IEEE Spectrum October, 2012 - 19
IEEE Spectrum October, 2012 - 20
IEEE Spectrum October, 2012 - 21
IEEE Spectrum October, 2012 - 22
IEEE Spectrum October, 2012 - 23
IEEE Spectrum October, 2012 - 24
IEEE Spectrum October, 2012 - 25
IEEE Spectrum October, 2012 - 26
IEEE Spectrum October, 2012 - 27
IEEE Spectrum October, 2012 - 28
IEEE Spectrum October, 2012 - 29
IEEE Spectrum October, 2012 - 30
IEEE Spectrum October, 2012 - 31
IEEE Spectrum October, 2012 - 32
IEEE Spectrum October, 2012 - 33
IEEE Spectrum October, 2012 - 34
IEEE Spectrum October, 2012 - 35
IEEE Spectrum October, 2012 - 36
IEEE Spectrum October, 2012 - 37
IEEE Spectrum October, 2012 - 38
IEEE Spectrum October, 2012 - 39
IEEE Spectrum October, 2012 - 40
IEEE Spectrum October, 2012 - 41
IEEE Spectrum October, 2012 - 42
IEEE Spectrum October, 2012 - 43
IEEE Spectrum October, 2012 - 44
IEEE Spectrum October, 2012 - 45
IEEE Spectrum October, 2012 - 46
IEEE Spectrum October, 2012 - 47
IEEE Spectrum October, 2012 - 48
IEEE Spectrum October, 2012 - 49
IEEE Spectrum October, 2012 - 50
IEEE Spectrum October, 2012 - 51
IEEE Spectrum October, 2012 - 52
IEEE Spectrum October, 2012 - 53
IEEE Spectrum October, 2012 - 54
IEEE Spectrum October, 2012 - 55
IEEE Spectrum October, 2012 - 56
IEEE Spectrum October, 2012 - 57
IEEE Spectrum October, 2012 - 58
IEEE Spectrum October, 2012 - 59
IEEE Spectrum October, 2012 - 60
IEEE Spectrum October, 2012 - 61
IEEE Spectrum October, 2012 - 62
IEEE Spectrum October, 2012 - 63
IEEE Spectrum October, 2012 - 64
IEEE Spectrum October, 2012 - 65
IEEE Spectrum October, 2012 - 66
IEEE Spectrum October, 2012 - 67
IEEE Spectrum October, 2012 - 68
IEEE Spectrum October, 2012 - 69
IEEE Spectrum October, 2012 - 70
IEEE Spectrum October, 2012 - 71
IEEE Spectrum October, 2012 - 72
IEEE Spectrum October, 2012 - 73
IEEE Spectrum October, 2012 - 74
IEEE Spectrum October, 2012 - 75
IEEE Spectrum October, 2012 - 76
IEEE Spectrum October, 2012 - 77
IEEE Spectrum October, 2012 - 78
IEEE Spectrum October, 2012 - 79
IEEE Spectrum October, 2012 - 80
IEEE Spectrum October, 2012 - Cover3
IEEE Spectrum October, 2012 - Cover4
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1217
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1117
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1017
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0917
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0817
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0717
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0617
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0517
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0417
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0317
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0217
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0117
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1216
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1116
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1016
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0916
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0816
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0716
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0616
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0516
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0416
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0316
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0216
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0116
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1215
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1115
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1015
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0915
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0815
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0715
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0615
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0515
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0415
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0315
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0215
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0115
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1214
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1114
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1014
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0914
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0814
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0714
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0614
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0514
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0414
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0314
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0214
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0114
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1213
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1113
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1013
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0913
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0813
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0713
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0613
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0513
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0413
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0313
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0213
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0113
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1212
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1112
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1012
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0912
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0812
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0712
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0612
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0512
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0412
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0312
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0212
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0112
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1211
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1111
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1011
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0911
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0811
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0711
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0611
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0511
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0411
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0311
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0211
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0111
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1210
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1110
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1010
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0910
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0810
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0710
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0610
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0510
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0410
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0310
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0210
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0110
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1209
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1109
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1009
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0909
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0809
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0709
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0609
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0509
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0409
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0309
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0209
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0109
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1208
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1108
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1008
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0908
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0808
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0708
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0608
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0508
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0408
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0308
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0208
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0108
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1207
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1107
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_1007
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0907
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0807
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0707
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0607
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0507
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0407
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0307
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0207
https://www.nxtbook.com/nxtbooks/ieee/spectrum_na_0107
https://www.nxtbookmedia.com