Editing Cosmic Background Explorer (section)

== Scientific findings ==
[[File:COBE cmb fluctuations.png|thumb|upright=1.0|right|The map of the CMB anisotropy formed from data taken by the COBE spacecraft.]]

The science mission was conducted by the three instruments detailed previously: DIRBE, FIRAS and DMR. The instruments overlapped in wavelength coverage, providing consistency check on measurements in the regions of spectral overlap and assistance in discriminating signals from our galaxy, [[Solar System]] and CMB.<ref name=boggess/>

COBE's instruments would fulfill each of their objectives as well as making observations that would have implications outside COBE's initial scope.

=== Black-body curve of CMB ===
[[File:Cmbr.svg|thumb|upright=1.0|left|Data from COBE showed a perfect fit between the black body curve predicted by big bang theory and that observed in the microwave background. The error bars are smaller than the width of the blue curve. The red crosses merely locate the data points.]]
[[File:PIA16874-CobeWmapPlanckComparison-20130321.jpg|thumb|upright=1.0|right|Comparison of [[CMB]] results from COBE, [[Wilkinson Microwave Anisotropy Probe|WMAP]] and [[Planck (spacecraft)|Planck]] - 21 March 2013.]]

During the 15-year-long period between the proposal and launch of COBE, there were two significant astronomical developments:
* First, in 1981, two teams of astronomers, one led by David Wilkinson of [[Princeton University]] and the other by Francesco Melchiorri of the [[University of Florence]], simultaneously announced that they detected a [[quadrupole]] distribution of CMB using balloon-borne instruments. This finding would have been the detection of the black-body distribution of CMB that FIRAS on COBE was to measure. In particular, the Florence group claimed a detection of intermediate angular scale anisotropies at the level 100 [[Orders of magnitude (temperature)|microkelvins]]<ref>{{cite journal |url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1981ApJ...250L...1M&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf |first1=Francesco |last1=Melchiorri |first2=Bianca O. |last2=Melchiorri |last3=Pietranera |first3=Luca |title=Fluctuations in the microwave background at intermediate angular scales |journal=The Astrophysical Journal |date=November 1981 |bibcode=1981ApJ...250L...1M |access-date=2011-08-23 |doi=10.1086/183662 |last4=Melchiorri |first4=B. O. |volume=250 |page=L1}}</ref> in agreement with later measurements made by the [[BOOMERanG experiment]]. However, a number of other experiments attempted to duplicate their results and were unable to do so.<ref name=leverington/>
* Second, in 1987 a Japanese-American team led by [[Andrew E. Lange]] and Paul Richards of [[University of California, Berkeley]] and Toshio Matsumoto of [[Nagoya University]] made an announcement that CMB was not that of a true black body.<ref>{{cite journal |title=Cosmological implication of a new measurement of the submillimeter background radiation |author=Hayakawa, S. |author2=Matsumoto, T. |author3=Matsuo, H. |author4=Murakami, Hiroshi |author5=Sato, S. |author6=Lange A. E. |author7=Richards, P. |name-list-style=amp |journal=Publications of the Astronomical Society of Japan |issn=0004-6264 |volume=39 |issue=6 |date=1987 |pages=941–948 |bibcode=1987PASJ...39..941H |url=http://articles.adsabs.harvard.edu//full/1987PASJ...39..941H/0000941.000.html |access-date=17 May 2012}}</ref> In a [[sounding rocket]] experiment, they detected an excess brightness at 0.5 and {{cvt|0.7|mm}} wavelengths.

With these developments serving as a backdrop to COBE's mission, scientists eagerly awaited results from FIRAS. The results of FIRAS were startling in that they showed a perfect fit of the CMB and the theoretical curve for a black body at a temperature of 2.7 K, in contrast to the Berkeley-Nagoya results.

FIRAS measurements were made by measuring the spectral difference between a 7° patch of the sky against an internal black body. The interferometer in FIRAS covered between 2- and 95-cm−1 in two bands separated at 20-cm−1. There are two scan lengths (short and long) and two scan speeds (fast and slow) for a total of four different scan modes. The data were collected over a ten-month period.<ref name=fixsen>{{cite journal |author1=Fixsen, D. J. |author2=Cheng, E. S. |author3=Cottingham, D. A. |author4=Eplee, R. E. Jr. |author5=Isaacman, R. B. |author6=Mather, J. C. |author7=Meyer, S. S. |author8=Noerdlinger, P. D. |author9=Shafer, R. A. |author10=Weiss, R. |author11=Wright, E. L. |author12=Bennett, C. L. |author13=Boggess, N. W. |author14=Kelsall, T. |author15=Moseley, S. H. |author16=Silverberg, R. F. |author17=Smoot, G. F. |author18=Wilkinson, D. T. |date=1994 |title=Cosmic microwave background dipole spectrum measured by the COBE FIRAS instrument |bibcode=1994ApJ...420..445F |journal=Astrophysical Journal |volume=420 |issue=2 |pages=445–449 |doi=10.1086/173575}}</ref>

=== Intrinsic anisotropy of CMB ===
[[File:COBE DMR Image.PNG|thumb|upright=1.0|right|Data obtained at each of the three DMR frequencies — 31.5, 53, and 90 [[Hertz|GHz]] — following dipole subtraction.]]

The DMR was able to spend four years mapping the detectable anisotropy of cosmic background radiation as it was the only instrument not dependent on the dewar's supply of helium to keep it cooled. This operation was able to create full sky maps of the CMB by subtracting out galactic emissions and dipole at various frequencies. The cosmic microwave background fluctuations are extremely faint, only one part in 100,000 compared to the 2.73 K average temperature of the radiation field. The cosmic microwave background radiation is a remnant of the [[Big Bang]] and the fluctuations are the imprint of density contrast in the early universe. The density ripples are believed to have produced [[structure formation]] as observed in the universe today: clusters of galaxies and vast regions devoid of galaxies.<ref>{{Cite web |last=Dick |first=Steven J. |date=16 October 2006 |title=Voyages to the Beginning of Time |url=https://www.nasa.gov/exploration/whyweexplore/Why_We_24.html |access-date=2022-06-16 |website=nasa.gov |publisher=NASA |language=en |archive-date=16 June 2022 |archive-url=https://web.archive.org/web/20220616205611/https://www.nasa.gov/exploration/whyweexplore/Why_We_24.html |url-status=dead}} {{PD-notice}}</ref>

=== Detecting early galaxies ===
DIRBE also detected 10 new far-IR emitting galaxies in the region not surveyed by IRAS as well as nine other candidates in the weak far-IR that may be [[spiral galaxies]]. Galaxies that were detected at the 140 and 240 μm were also able to provide information on very cold dust (VCD). At these wavelengths, the mass and temperature of VCD can be derived. When these data were joined with 60 and 100 μm data taken from IRAS, it was found that the far-infrared luminosity arises from cold (≈17–22 K) dust associated with diffuse [[H I region]] cirrus clouds, 15-30% from cold (≈19 K) dust associated with molecular gas, and less than 10% from warm (≈29 K) dust in the extended low-density [[H II region]]s.<ref name=sodroski>{{cite journal |author=T. J. Sodroski |title=Large-Scale Characteristics of Interstellar Dust from COBE DIRBE Observations |bibcode=1994ApJ...428..638S |volume=428 |issue=2 |pages=638–646 |date=1994 |doi=10.1086/174274 |journal=The Astrophysical Journal |doi-access=free}}</ref>

=== DIRBE ===
{{Main|Diffuse Infrared Background Experiment}}
[[File:COBE galactic disk.PNG|thumb|upright=1.0|right|Model of the Galactic disk as seen edge-on from Earth's position.]]

On top of the findings DIRBE had on galaxies, it also made two other significant contributions to science.<ref name=sodroski/> The DIRBE instrument was able to conduct studies on [[interplanetary dust]] (IPD) and determine if its origin was from asteroid or cometary particles. The DIRBE data collected at 12, 25, 50 and 100 μm were able to conclude that grains of [[asteroid]]al origin populate the IPD bands and the smooth IPD cloud.<ref name=spiesman>{{cite journal |author1=Spiesman, W. J. |author2=M. G. Hauser |author3=T. Kelsall |author4=C. M. Lisse |author5=S. H. Moseley Jr. |author6=W. T. Reach |author7=R. F. Silverberg |author8=S. W. Stemwedel |author9=J. L. Weiland |name-list-style=amp |date=1995 |title=Near and far-infrared observations of interplanetary dust bands from the COBE Diffuse Infrared Background Experiment |bibcode=1995ApJ...442..662S |journal=Astrophysical Journal |volume=442 |issue=2 |page=662 |doi=10.1086/175470 |doi-access=free}}</ref>

The second contribution DIRBE made was a model of the [[Galactic disk]] as seen edge-on from our position. According to the model, if the Sun is 8.6 [[Parsec|kpc]] from the [[Galactic Center]], then it is 15.6% above the midplane of the disk, which has a radial and vertical scale lengths of 2.64 and 0.333 kpc, respectively, and is warped in a way consistent with the HI layer. There is also no indication of a thick disk.<ref name=freudenreich>{{cite journal |author=Freudenreich, H. T. |date=1996 |title=The shape and color of the galactic disk |bibcode=1996ApJ...468..663F |journal=Astrophysical Journal |volume=468 |pages=663–678 |doi=10.1086/177724 |doi-access=free}} See also {{cite journal |author=Freudenreich, H. T. |date=1997 |title=The shape and color of the galactic disk: Erratum |bibcode=1997ApJ...485..920F |journal=Astrophysical Journal |volume=485 |issue=2 |page=920 |doi=10.1086/304478 |doi-access=free}}</ref>

To create this model, the IPD had to be subtracted out of the DIRBE data. It was found that this cloud, which as seen from Earth is [[Zodiacal light]], was not centered on the Sun, as previously thought, but on a place in space a few million kilometers away. This is due to the gravitation influence of [[Saturn]] and [[Jupiter]].<ref name=leverington/>

=== Cosmological implications ===
In addition to the science results detailed in the last section, there are numerous cosmological questions left unanswered by COBE's results. A direct measurement of the [[extragalactic background light]] (EBL) can also provide important constraints on the integrated cosmological history of star formation, metal and dust production, and the conversion of starlight into infrared emissions by dust.<ref name=dwek>{{cite journal |author=Dwek, E. |author2=R. G. Arendt |author3=M. G. Hauser |author4=D. Fixsen |author5=T. Kelsall |author6=D. Leisawitz |author7=Y. C. Pei |author8=E. L. Wright |author9=J. C. Mather |author10=S. H. Moseley |author11=N. Odegard |author12=R. Shafer |author13=R. F. Silverberg |author14=J. L. Weiland |name-list-style=amp |date=1998 |title=The COBE Diffuse Infrared Background Experiment search for the cosmic infrared background: IV. Cosmological Implications |bibcode=1998ApJ...508..106D |journal=Astrophysical Journal |volume=508 |issue=1 |pages=106–122 |doi=10.1086/306382 |arxiv=astro-ph/9806129 |s2cid=14706133}}</ref>

By looking at the results from DIRBE and FIRAS in the 140 to 5000 μm we can detect that the integrated EBL intensity is ≈16 [[Watt|nW]]/(m<sup>2</sup>·sr). This is consistent with the energy released during [[nucleosynthesis]] and constitutes about 20–50% of the total energy released in the formation of [[helium]] and metals throughout the history of the universe. Attributed only to nuclear sources, this intensity implies that more than 5–15% of the baryonic mass density implied by Big Bang nucleosynthesis analysis has been processed in stars to helium and heavier elements.<ref name=dwek/>

There were also significant implications into [[star formation]]. COBE observations provide important constraints on the cosmic star formation rate and help us calculate the EBL spectrum for various star formation histories. Observation made by COBE require that star formation rate at redshifts of ''z'' ≈ 1.5 to be larger than that inferred from UV-optical observations by a factor of 2. This excess stellar energy must be mainly generated by massive stars in yet - undetected dust enshrouded galaxies or extremely dusty star-forming regions in observed galaxies.<ref name=dwek/> The exact star formation history cannot unambiguously be resolved by COBE and further observations must be made in the future.

On 30 June 2001, NASA launched a follow-up mission to COBE led by DMR Deputy Principal Investigator [[Charles L. Bennett]]. The [[Wilkinson Microwave Anisotropy Probe]] has clarified and expanded upon COBE's accomplishments. Following WMAP, the European Space Agency's probe, [[Planck (spacecraft)|Planck]] has continued to increase the resolution at which the background has been mapped.<ref>{{cite web |last=Thomas |first=Christopher |title=Planck's Probe Map - A picture of the Universe |date=3 April 2013 |url=http://dawn.com/2013/04/03/plank-probes-map-a-picture-of-our-universe/ |publisher=Spider Magazine |access-date=28 May 2013}}</ref><ref>{{cite web |title=Planck's HFI Completes Its Survey of the Early Universe |url=http://www.esa.int/Our_Activities/Space_Science/Planck/Planck_s_HFI_completes_its_survey_of_early_Universe |publisher=ESA |access-date=28 May 2013}}</ref>