U.S. Department of Energy

Office of Science

For more information on the Office of Science visit our website:

www.science.doe.gov

Dark Energy—the Mystery that Dominates the Universe

Summary

Recently scientists sponsored by the Office of Science found that, contrary to all previous 

understanding, the expansion of the universe was accelerating; some force was pushing galaxies 

apart at ever increasing speed. The study of this force—now called “Dark Energy”—holds the 

promise of a new understanding of the fundamental physical laws that govern our universe.

Several years ago, a group of physicists at the 

DOE Office of Science’s Lawrence Berkeley 

National Laboratory set out to measure how 

gravitational attraction was slowing down the 

expansion of the universe that started with the 

Big Bang. Would gravity be enough to reverse 

the expansion and cause the universe to end in a 

big crunch when everything collapsed like a 

deflated balloon? Or is space growing so fast 

that gravity can’t contain it? 

How is it possible to answer this huge question? 

The Berkeley Lab scientists showed they could 

use the brightness of distant exploding stars—

supernovae—as natural beacons from the past. 

They measured the brightness of many 

thousands of galaxies at a time and then 

measured them again after waiting a few weeks 

so that a dozen or so new supernovae explosions 

would show themselves as brightening spots 

amidst these galaxies. They selected only one 

kind of supernova, which is known always to 

explode with the same brightness. How bright 

such a supernova appears to us tells how far 

away it must have been at the time of the 

explosion. And the farther away they are, the 

farther back in time the explosion occurred, 

since it takes more time for the light to reach us. 

In the end they were able to find supernovae that 

had exploded 1 billion years ago, 2 billion years 

ago, 3 billion years ago...as far back as 7 billion 

years! 

While the light from the supernova is making its 

way to us, its wavelength expands as the 

universe itself expands. So measuring the 

wavelength of the supernova light directly 

reveals how much the universe has expanded 

since the time of the supernova explosion. After 

several years of supernova discoveries were 

collected, the scientists compared light from the 

more and more distant supernovae to see how 

much the expansion of the universe had been 

slowing down.

When the results were in, the answer was 

shocking. The universe wasn’t slowing down at 

all, but rather speeding up, much as if the tossed 

ball defied gravity and picked up more and more 

speed as it left the earth behind. Apparently, 

some previously unknown energy is pushing the 

universe to expand faster and faster. Moreover, 

most of the universe—2/3 of it—is made of this 

mysterious “dark energy!”

Nothing of the sort had ever been seen before, 

but it had been imagined. Einstein’s Theory of 

General Relativity did permit such behavior 

when he included a “cosmological constant” in 

his equations. However, Einstein eventually 

rejected this possibility and declared it had been 

a mistake from the start, “his greatest blunder.” 

Maybe he was right the first time. 

Physicists have long doubted that there is a 

cosmological constant because all predicttions of 

its size, based on everything we know today 

about physics, come out wildly wrong. In fact, 

these predictions have been called the worst 

predictions ever made—like guessing something 

weighed as much as a flea when it actually 
U.S. Department of Energy

Office of Science

For more information on the Office of Science visit our website:

www.science.doe.gov

weighed as much as all the galaxies of all the 

stars and planets put together. 

A hard puzzle like this is just what makes for 

scientific revolution. One of the greatest 

physicists today, Nobel Prize winner Steven 

Weinberg, remarks: “The problem of the dark 

energy is central to today's physics. Until it is 

solved, the problem of the dark energy will be a 

roadblock on our path to a comprehensive 

fundamental physical theory.” 

But with great difficulties come great 

opportunities. At the beginning of the last 

century physics was stumped by discoveries that 

when explained led to atomic and quantum 

physics, with all the incredible technological and 

material gains brought by understanding 

particles, light, and the quantum. 

Jack Marburger, the President's Science Adviser, 

spoke about the recent discoveries: “By 

century’s end, everything had come aliveeven 

time and space itself. Our universe today is filled 

with cataclysmic change on the widest possible 

scales of energy and time. Its laws are deeper 

than we suspected, and more full of mystery 

than any other part of science except possibly 

for that which studies human consciousness. 

...[I]t appears that the heavens themselves will 

be the laboratory in which the final pieces of the 

puzzle, or at least the next ones, will be found.” 

How do we start on this new puzzle of dark 

energy? By going back to the supernova 

explosionsbut now studying them with much 

more precise instruments and with techniques 

only possible in outer space. The scientists at 

DOE’s Berkeley Lab have embarked on a new 

program to design, build, and launch a new 

Space Telescope, called the SuperNova / 

Acceleration Probe (SNAP). Like the Hubble 

Space Telescope, SNAP would have a mirror 

two meters across, but its wide-field camera 

with a billion pixels would collect images 

hundreds of times larger than those of its 

predecessor. SNAP would examine thousands 

of new supernovae with a precision unattainable 

from the ground, and in this way map out the 

details of the expansion history of our universe. 

Measuring spurts or slowdowns in this 

expansion history provides the only known way 

to study the dark energy.

With SNAP, we have the opportunity to explore 

the beginning and end of the universe through 

this elemental, fateful dark energy. Its 

“antigravity” properties are unlike anything ever 

seen before. Today’s established physics 

theories explain all aspects of nature, from 

material properties such as superconductivity to 

nuclear fission and fusionbut they don’t 

explain dark energy, the dominant stuff of the 

universe. 

The physics of a century ago similarly seemed 

just about complete, but the few inconsistencies 

at that time led to quantum physics, 

semiconductors, lasers, CAT scans, and nuclear

power, things too strange and wonderful to have 

been dreamt of. 

We don’t know where the puzzle of dark energy 

will lead, but we have the tools, the knowledge, 

and the ability to pursue this discovery made by 

DOE Office of Science research, to extend our 

reach, and hopefully our grasp, into new realms.

For further information on this subject contact:

Dr. Peter Rosen, Director

Office of High Energy and Nuclear Physics

301-903-3713

peter.rosen@science.doe.gov