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A brighter universe found: ours

May 16, 2008
Courtesy Science & Technology
Facilities Council, U.K.
and World Science staff

The Uni­ver­se is twice as bright as pre­vi­ously be­lieved: dust turns out to block about half the star­light from us, as­tro­no­mers have found.

“In­ter­stel­lar dust grains have a dev­as­tat­ing ef­fect on our mea­sure­ments of the en­er­gy out­put from even near­by ga­lax­ies,” said Rich­ard Tuffs of the Max Plank In­sti­tute for Nu­clear Phys­ics in Hei­del­berg, Ger­ma­ny, one of the re­search­ers.

The dusty gal­axy NGC 4565 (Cour­tesy Rob­ert Gen­d­ler)
As­tro­no­mers have long known there’s dust out there, but did­n’t know how much this re­stricts the light we can see. The dust it­self glows, be­cause it ab­sorbs and then re-emits star­light. 

Some­thing “very wrong” has af­flicted past the­o­ries touch­ing on the is­sue, said Si­mon Driv­er of the Uni­ver­s­ity of St An­drews, U.K., lead au­thor of a re­port on the find­ings. The pre­vi­ous mod­els, he con­tin­ued, show the glow­ing dust’s en­er­gy out­put as great­er than the stars’ to­tal en­er­gy, which is im­pos­si­ble.

His team as­sem­bled a high-resolution cat­a­logue of 10,000 ga­lax­ies and an­a­lyzed it to­geth­er with a new mod­el of ga­lac­tic dust dis­tri­bu­tion de­vel­oped by Tuffs and Cristina Po­pes­cu of the Uni­ver­s­ity of Cen­tral Lan­ca­shire, U.K.

With the new mod­el, the as­tro­no­mers said they could cal­cu­late the pre­cise frac­tion of star­light blocked. The ab­sorbed star­light en­er­gy fi­nally equal­led that de­tected from the glow­ing dust, as makes sense, they said. 

“For the first time we have a to­tal un­der­stand­ing of the en­er­gy out­put of the Uni­ver­se over a mon­u­men­tal wave­length [light en­er­gy] range,” said Po­p­escu. (Sci­ent­ists use the term “light” to in­clude not just vis­i­ble light but also the forms that are in­vi­si­ble to the eye be­cause their energy is lower or high­er than than what we na­tur­ally see.)

The team meas­ured cos­mic en­er­gy per cu­bic light-year—a cu­be with each side’s length the dis­tance light moves in a year. With­in such a space, the Uni­ver­se gen­er­ates some five quad­ril­lion Watts yearly on av­er­age, about 300 times the en­er­gy con­sump­tion of Earth’s popula­t­ion, the re­search­ers said. 

The find­ings ap­pear in the May 10 is­sue of the re­search pub­lica­t­ion As­t­ro­phys­i­cal Jour­nal Let­ters.

The team meas­ured the bright­ness of thou­sands of disc-shaped ga­lax­ies with dif­fer­ent ori­enta­t­ions, then matched the re­sults to com­put­er mod­els of dusty ga­lax­ies. Based on this they cal­i­brat­ed the mod­els to find out how much light is blocked when a gal­axy is seen face-on. This in turn let them de­ter­mine the frac­tion of ga­lac­tic light that es­capes in each di­rec­tion.

“For over 70 years an ac­cu­rate de­scrip­tion of how ga­lax­ies—the loca­t­ions where mat­ter is churned in­to en­er­gy—form and evolve has elud­ed us,” said Driv­er. “Bal­anc­ing the cos­mic ‘en­er­gy budg­et’ is an im­por­tant step for­ward.”

* * *

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The Universe is twice as bright as previously thought: dust turns out to block about half the starlight from our view, astronomers have found. “Interstellar dust grains have a devastating effect on our measurements of the energy output from even nearby galaxies” said Richard Tuffs of the Max Plank Institute for Nuclear Physics in Heidelberg, Germany, one of the researchers. Astronomers have long known there’s dust out there, but didn’t know how much this restricts the light we can see. The dust itself glows, because it absorbs and then re-emits starlight. Something “very wrong” has afflicted past theories touching on the issue, said Simon Driver of the University of St Andrews, U.K., lead author of the report on the new findings. The previous models, he continued, show the glowing dust’s energy output as greater than the stars’ total energy, which is impossible. His team assembled a high-resolution catalogue of 10,000 galaxies and analyzed it together with a new model of galactic dust distribution developed by Tuffs and Cristina Popescu of the University of Central Lancashire, U.K. With the new model, the astronomers said they could calculate the precise fraction of starlight blocked. The absorbed starlight energy finally equalled that detected from the glowing dust, as makes sense, they said. “For the first time we have a total understanding of the energy output of the Universe over a monumental wavelength [light energy] range,” said Popescu. The team measured cosmic energy per cubic light-year, a cube with each side’s length the distance light moves in a year. Within such a space, the Universe generates some five quadrillion Watts yearly on average, about 300 times the energy consumption of Earth’s population, the researchers said. The findings appear in the May 10 issue of the research publication Astrophysical Journal Letters. The team measured the brightness of thousands of disc-shaped galaxies with different orientations, then matched the results to computer models of dusty galaxies. Based on this they calibrated the models to find out how much light is blocked when a galaxy is seen face-on. This in turn let them determine the fraction of galactic light that escapes in each direction. “For over 70 years an accurate description of how galaxies, the locations where matter is churned into energy, form and evolve has eluded us. Balancing the cosmic energy budget is an important step forward,” said Driver.