3064:
688:
928:
3076:
552:
29:
3112:
3136:
3088:
3124:
3100:
323:
651:, and a mean lifetime of the excited state of around 11 million years. Collisions of neutral hydrogen atoms with electrons or other atoms can help promote the emission of 21 cm photons. A spontaneous occurrence of the transition is unlikely to be seen in a laboratory on Earth, but it can be artificially induced through
906:, so they must be made from very secluded sites with care taken to eliminate interference. Space based experiments, even on the far side of the Moon (where they would be sheltered from interference from terrestrial radio signals), have been proposed to compensate for this. Little is known about other foreground effects, such as
547:{\displaystyle \lambda ={\frac {1}{\nu }}\cdot c={\frac {h}{E}}\cdot c\approx {\frac {\;4.135\,67\cdot 10^{-15}\ \mathrm {eV} \cdot {\text{s}}\;}{5.874\,33\cdot 10^{-6}\ \mathrm {eV} }}\,\cdot \,2.997\,92\cdot 10^{8}\ \mathrm {m} \cdot \mathrm {s} ^{-1}\approx 0.211\,06\ \mathrm {m} =21.106\ \mathrm {cm} \;}
947:
spacecraft, portrays the hyperfine transition of neutral hydrogen and used the wavelength as a standard scale of measurement. For example, the height of the woman in the image is displayed as eight times 21 cm, or 168 cm. Similarly the frequency of the hydrogen spin-flip transition was used
297:
just as two magnets will align so that the north pole of one is closest to the south pole of the other. This logic fails here because the wave functions of the electron and the proton overlap; that is, the electron is not spatially displaced from the proton, but encompasses it. The magnetic dipole
980:
published "Searching for interstellar communications", a paper proposing the 21 cm hydrogen line and the potential of microwaves in the search for interstellar communications. According to George
Basalla, the paper by Cocconi and Morrison "provided a reasonable theoretical basis" for the
2931:
Wang, Jingying; Xu, Haiguang; An, Tao; Gu, Junhua; Guo, Xueying; Li, Weitian; Wang, Yu; Liu, Chengze; Martineau-Huynh, Olivier; Wu, Xiang-Ping (2013-01-14). "Exploring the cosmic reionization epoch in frequency space: An improved approach to remove the foreground in 21 cm tomography".
810:. Assuming that the hydrogen atoms are uniformly distributed throughout the galaxy, each line of sight through the galaxy will reveal a hydrogen line. The only difference between each of these lines is the Doppler shift that each of these lines has. Hence, by assuming
822:
hydrogen line. It is then possible to use the plot of the rotation curve and the velocity to determine the distance to a certain point within the galaxy. However, a limitation of this method is that departures from circular motion are observed at various scales.
701:
During the 1930s, it was noticed that there was a radio "hiss" that varied on a daily cycle and appeared to be extraterrestrial in origin. After initial suggestions that this was due to the Sun, it was observed that the radio waves seemed to propagate from the
960:
whose rotation period circa 1977 is given as a multiple of the frequency of the hydrogen spin-flip transition. It is theorized by the plaque's creators that an advanced civilization would then be able to use the locations of these pulsars to locate the
857:
Deuterium has a similar hyperfine spectral line at 91.6 cm (327 MHz), and the relative strength of the 21 cm line to the 91.6 cm line can be used to measure the deuterium-to-hydrogen (D/H) ratio. One group in 2007 reported D/H ratio in the
1735:
Skywaves are not used in the UHF band because the ionosphere is not sufficiently dense to reflect the waves, which pass through it into space. ... Reception of UHF signals is virtually free from fading and interference from atmospheric
898:
in the period after recombination. Second, it can provide a picture of how the universe was re‑ionized, as neutral hydrogen which has been ionized by radiation from stars or quasars will appear as holes in the 21 cm background.
982:
797:
to be exact). Electromagnetic energy in this range can easily pass through the Earth's atmosphere and be observed from the Earth with little interference. The hydrogen line can readily penetrate clouds of interstellar
914:
on the galaxy. Despite these problems, 21 cm observations, along with space-based gravitational wave observations, are generally viewed as the next great frontier in observational cosmology, after the
284:, whose interaction results in a slight increase in energy when the spins are parallel, and a decrease when antiparallel. The fact that only parallel and antiparallel states are allowed is a result of the
1145:, so it is pronounced "H one". It is also called the "neutral hydrogen" line, with the listener obliged to infer from context that "cold neutral hydrogen" at 1420.4 MHz or 0.211 m is meant.
292:
of the system. When the spins are parallel, the magnetic dipole moments are antiparallel (because the electron and proton have opposite charge), thus one would expect this configuration to actually have
2776:
Loeb, A.; Zaldarriaga, M. (2004). "Measuring the small-scale power spectrum of cosmic density fluctuations through 21 cm tomography prior to the epoch of structure formation".
902:
However, 21 cm observations are very difficult to make. Ground-based experiments to observe the faint signal are plagued by interference from television transmitters and the
298:
moments are therefore best thought of as tiny current loops. As parallel currents attract, the parallel magnetic dipole moments (i.e., antiparallel spins) have lower energy.
32:
A hydrogen atom with proton and electron spins aligned (top) undergoes a flip of the electron spin, resulting in emission of a photon with a 21 cm wavelength (bottom)
1251:
Mhaske, Ashish A.; et al. (August 2022). "A Bose horn antenna radio telescope (BHARAT) design for 21 cm hydrogen line experiments for radio astronomy teaching".
1655:
van de Hulst, H. C.; et al. (May 1954). "The spiral structure of the outer part of the
Galactic System derived from the hydrogen emission at 21 cm wavelength".
1913:
Verschuur, G. L. (September 1968). "Positive
Determination of an Interstellar Magnetic Field by Measurement of the Zeeman Splitting of the 21-cm Hydrogen Line".
764:
and
Hindman in Australia. After 1952 the first maps of the neutral hydrogen in the Galaxy were made, and revealed for the first time the spiral structure of the
3009:
Pen, Ue-Li; Wu, Xiang-Ping; Peterson, Jeff (5 April 2004). "Forecast for Epoch-of-Reionization as viewable by the PrimevAl
Structure Telescope (PAST)".
2212:
Kohler, Katharina; et al. (November 2005). "Redshifted 21 cm
Emission from the Pre-Reionization Era. II. H II Regions around Individual Quasars".
2725:
1077:, and would clearly signify its artificial origin. Such a signal would not be overwhelmed by the H I line itself, or by any of its harmonics.
1294:
890:, this line will be observed at frequencies from 200 MHz to about 15 MHz on Earth. It potentially has two applications. First, by
1716:
1610:
Muller, C.A.; Oort, J.H. (September 1951). "The interstellar hydrogen line at 1,420 Mc./sec., and an estimate of galactic rotation".
916:
826:
Hydrogen line observations have been used indirectly to calculate the mass of galaxies, to put limits on any changes over time of the
243:
1565:(September 1951). "Observation of a line in the galactic radio spectrum: Radiation from galactic hydrogen at 1,420 Mc./sec".
2267:"Transformative science from the lunar farside: observations of the dark ages and exoplanetary systems at low radio frequencies"
2833:
Santos, M.G.; Cooray, A.; Knox, L. (2005). "Multifrequency analysis of 21 cm fluctuations from the era of reionization".
2044:
Peters, Wendy M.; et al. (January 2011). "Radio
Recombination Lines at Decameter Wavelengths: Prospects for the Future".
2526:
2453:
67:-flip transition, which means the direction of the electron's spin is reversed relative to the spin of the proton. This is a
2668:
Zaldarriaga, M.; Furlanetto, S.; Hernquist, L. (2004). "21 centimeter fluctuations from cosmic gas at high redshifts".
3001:
289:
242:
because it can be used to study the early
Universe. Due to its fundamental properties, this line is of interest in the
2617:
Ciardi, B.; Madau, P. (2003). "Probing beyond the epoch of hydrogen reionization with 21 centimeter radiation".
1759:
2882:
Barkana, R.; Loeb, A. (2005). "Detecting the earliest galaxies through two new sources of 21 cm fluctuations".
3063:
2566:
Madau, P.; Meiksin, A.; Rees, M.J. (1997). "21 cm tomography of the intergalactic medium at high redshift".
3161:
3033:
972:
program in their search for signals from potential extraterrestrial civilizations. In 1959, Italian physicist
134:
2155:"Simulating cosmic reionization at large scales - II. The 21-cm emission features and statistical signals"
3171:
1213:
172:
2476:"Introducing Humans to the Extraterrestrials: the Pioneering Missions of the Pioneer and Voyager Probes"
2097:
Fialkov, A.; Loeb, A. (November 2013). "The 21-cm Signal from the cosmological epoch of recombination".
3054:
1092:
894:
of redshifted 21 centimeter radiation it can, in principle, provide a very precise picture of the
761:
3156:
2934:
2884:
2835:
2670:
2619:
2568:
1400:
1174:
879:
86:
3166:
1396:
1121:
1102:
1097:
195:
79:
76:
20:
3176:
2778:
2514:
827:
235:
2518:
2508:
672:
851:
760:, and published after their data was corroborated by Dutch astronomers Muller and Oort, and by
277:
593:. In a laboratory setting, the hydrogen line parameters have been more precisely measured as:
1749:
1685:(May 1957). "The distribution of atomic hydrogen in the outer parts of the Galactic System".
907:
895:
875:
664:
2067:
1791:
234:, to calculate the mass and dynamics of individual galaxies, and to test for changes to the
230:
in 1951. Observations of the hydrogen line have been used to reveal the spiral shape of the
2953:
2903:
2854:
2797:
2744:
2689:
2638:
2587:
2408:
2351:
2288:
2231:
2176:
2116:
2063:
2002:
1961:
1950:"The 21 cm absorption line profile as a tool for the search for antimatter in the universe"
1922:
1877:
1826:
1787:
1694:
1664:
1621:
1576:
1562:
1523:
1481:
1415:
1360:
1303:
1222:
1179:
790:
753:
715:
223:
219:
57:
687:
8:
3140:
1990:
859:
843:
652:
259:
2957:
2907:
2858:
2801:
2748:
2693:
2642:
2591:
2412:
2355:
2292:
2235:
2180:
2128:
2120:
2006:
1965:
1926:
1881:
1830:
1799:
1698:
1668:
1625:
1580:
1493:
1485:
1419:
1364:
1307:
1226:
1183:
931:
The hyperfine transition of hydrogen, as depicted on the
Pioneer and Voyager spacecraft.
3128:
3116:
3010:
2977:
2943:
2919:
2893:
2870:
2844:
2821:
2787:
2762:
2734:
2705:
2679:
2654:
2628:
2603:
2577:
2429:
2398:
2386:
2367:
2341:
2309:
2278:
2266:
2247:
2221:
2194:
2166:
2132:
2106:
2079:
2053:
2026:
1895:
1890:
1867:
1855:
1712:
1637:
1592:
1543:
1505:
1471:
1431:
1378:
1350:
1260:
835:
757:
749:
227:
2997:
3181:
2981:
2969:
2965:
2813:
2757:
2720:
2709:
2607:
2522:
2434:
2371:
2314:
2189:
2154:
2136:
2018:
1755:
1724:
The
American Practical Navigator: An Epitome of Navigation. 2002 Bicentennial Edition
1497:
1435:
1373:
1338:
1319:
1070:
891:
285:
2874:
2825:
2658:
2251:
2198:
2083:
2030:
1641:
1596:
1509:
1427:
1382:
694:
used by Ewen and Purcell for the first detection of hydrogen line emission from the
3092:
3080:
2961:
2923:
2911:
2862:
2805:
2766:
2752:
2697:
2646:
2595:
2487:
2424:
2416:
2359:
2304:
2296:
2239:
2184:
2124:
2071:
2010:
1969:
1930:
1899:
1885:
1834:
1795:
1629:
1612:
1584:
1567:
1535:
1489:
1423:
1368:
1311:
1270:
1230:
1187:
973:
803:
794:
211:
72:
2809:
2075:
1289:
948:
for a unit of time in a map to Earth included on the Pioneer plaques and also the
1775:
1682:
1116:
1028:
977:
811:
719:
703:
582:
314:
302:
199:
180:
2332:
Wang, Xiaomin; et al. (October 2006). "21 cm Tomography with Foregrounds".
3068:
2420:
1934:
1211:
Griffiths, D.J. (1982). "Hyperfine splitting in the ground state of hydrogen".
936:
911:
831:
815:
782:
656:
640:
590:
281:
247:
64:
2492:
2475:
956:
probes. On this map, the position of the Sun is portrayed relative to 14
3150:
2973:
2022:
1854:
Drinkwater, M. J.; Webb, J. K–; Barrow, J. D.; Flambaum, V. V. (April 1998).
1323:
1191:
1142:
1087:
839:
807:
742:
711:
676:
668:
215:
151:
68:
60:
53:
49:
1290:"Accurate atomic transition probabilities for hydrogen, helium, and lithium"
710:
who knew that significant advances could be made in astronomy if there were
3104:
2817:
2438:
2318:
2300:
1501:
962:
883:
847:
738:
691:
273:
167:
82:
1974:
1949:
927:
3015:
2898:
2849:
2792:
2739:
2684:
2633:
2582:
2346:
2226:
2171:
1872:
1355:
1339:"The spin temperature of neutral hydrogen during cosmic pre-reionization"
1166:
922:
799:
272:. The lowest stationary energy state of the bound electron is called its
207:
1547:
1167:"Measurement of the unperturbed hydrogen hyperfine transition frequency"
1073:, such a frequency could not possibly be produced in a natural way as a
968:
The 21 cm hydrogen line is considered a favorable frequency by the
2721:"Observing the reionization epoch through 21 centimeter radiation"
2387:"Peering into the dark (ages) with low-frequency space interferometers"
944:
940:
903:
660:
562:
203:
112:
3008:
1315:
1274:
874:
cosmology because it is the only known way to probe the cosmological "
814:, one can calculate the relative speed of each arm of our galaxy. The
679:
caused by bulk motion or nonzero temperature of the emitting regions.
1633:
1588:
1539:
1111:
953:
949:
765:
726:
695:
570:
239:
231:
191:
107:
90:
1234:
748:
The 21 cm line (1420.4 MHz) was first detected in 1951 by
28:
3037:
2915:
2866:
2701:
2650:
2599:
2403:
2363:
2283:
2243:
2014:
1839:
1814:
1459:
1265:
1074:
887:
871:
722:
707:
265:
16:
Spectral line of hydrogen state transition in UHF radio frequencies
2998:"The story of Ewen and Purcell's discovery of the 21 cm line"
2948:
2543:
2111:
2058:
1778:(1969). "The Large-Scale Distribution of Hydrogen in the Galaxy".
1476:
1107:
2211:
3045:
1110:, the visible red spectral line with wavelength of 656.28
957:
786:
269:
218:. The existence of this line was predicted by Dutch astronomer
187:
147:
130:
2667:
1989:
Rogers, A. E. E.; Dudevoir, K. A.; Bania, T. M. (2007-03-09).
2384:
706:. These discoveries were published in 1940 and were noted by
1991:"Observations of the 327 MHz Deuterium Hyperfine Transition"
1856:"New limits on the possible variation of physical constants"
1853:
1164:
2718:
969:
659:. It is commonly observed in astronomical settings such as
3099:
1687:
Bulletin of the Astronomical Institutes of the Netherlands
1657:
Bulletin of the Astronomical Institutes of the Netherlands
842:
on the 21-cm line; a task that was first accomplished by
305:
between these aligned states has an energy difference of
3025:
1528:
Proceedings of the American Academy of Arts and Sciences
830:, and to study the dynamics of individual galaxies. The
2043:
1064:
985:
proposed SETI use a frequency which is equal to either
714:
in the radio part of the spectrum. He referred this to
2152:
1815:"Integral Properties of Spiral and Irregular Galaxies"
1654:
1462:(August 2012). "21 cm cosmology in the 21st century".
1250:
923:
Relevance to the search for non-human intelligent life
3052:
2454:"The Pioneer Plaque: Science as a Universal Language"
2331:
2148:
2146:
1948:
Solovyev, Dmitry; Labzowsky, Leonti (November 2014).
1453:
1451:
846:
in 1968. In theory, it may be possible to search for
326:
2719:
Furlanetto, S.; Sokasian, A.; Hernquist, L. (2004).
1175:
IEEE Transactions on Instrumentation and Measurement
1988:
1246:
1244:
52:that is created by a change in the energy state of
2143:
1726:. National Imagery and Mapping Agency. p. 158
1448:
546:
276:. Both the electron and the proton have intrinsic
2726:Monthly Notices of the Royal Astronomical Society
2271:Philosophical Transactions of the Royal Society A
2159:Monthly Notices of the Royal Astronomical Society
2090:
1947:
1860:Monthly Notices of the Royal Astronomical Society
1343:Monthly Notices of the Royal Astronomical Society
854:of the 21-cm line in an external magnetic field.
3148:
2832:
2565:
1954:Progress of Theoretical and Experimental Physics
1241:
781:The 21 cm spectral line appears within the
2775:
2385:Koopmans, Léon V. E.; et al. (June 2021).
1295:Journal of Physical and Chemical Reference Data
737:due to two closely spaced energy levels in the
206:can penetrate the large clouds of interstellar
2153:Mellema, Garrelt; et al. (October 2006).
2099:Journal of Cosmology and Astroparticle Physics
1747:
1457:
882:(when stable hydrogen atoms first formed) to
238:over time. It is of particular importance to
818:of our galaxy has been calculated using the
246:. This line is the theoretical basis of the
2881:
2616:
2096:
1780:Annual Review of Astronomy and Astrophysics
1560:
1526:(1953). "Line Spectra in Radio Astronomy".
643:with an extremely small transition rate of
264:An atom of neutral hydrogen consists of an
190:, which is located in the lower end of the
2930:
1754:. Cambridge University Press. p. 36.
1681:
1609:
965:at the time the spacecraft were launched.
543:
417:
374:
111:(1.42 GHz), which is equivalent to a
3014:
2947:
2897:
2848:
2791:
2756:
2738:
2683:
2632:
2581:
2491:
2451:
2428:
2402:
2345:
2308:
2282:
2225:
2188:
2170:
2110:
2057:
1973:
1912:
1889:
1871:
1838:
1475:
1372:
1354:
1287:
1264:
1210:
663:in our galaxy and others. Because of the
514:
465:
461:
457:
423:
378:
2546:[Look at the root] (in Russian).
1711:
926:
917:cosmic microwave background polarization
686:
244:search for extraterrestrial intelligence
186:The hydrogen line frequency lies in the
27:
2506:
2474:Capova, Klara Anna (October 18, 2021).
1812:
1522:
3149:
2541:
2473:
1395:
1336:
1288:Wiese, W.L.; Fuhr, J.R. (2009-06-24).
981:then-nascent SETI program. Similarly,
2264:
1813:Roberts, Morton S. (September 1969).
1165:Hellwig, Helmut; et al. (1970).
776:
3034:"Mileura Widefield Array experiment"
3002:National Radio Astronomy Observatory
2452:Rosenthal, Jake (January 20, 2016).
1774:
3046:"Square kilometer array experiment"
1800:10.1146/annurev.aa.07.090169.000351
222:in 1944, then directly observed by
19:For hydrogen lines in general, see
13:
2553:
1891:10.1046/j.1365-8711.1998.2952457.x
539:
536:
522:
495:
486:
450:
447:
405:
402:
150:emitted by this transition has an
14:
3193:
2990:
870:The line is of great interest in
838:can be measured by observing the
3134:
3122:
3110:
3098:
3086:
3074:
3062:
2758:10.1111/j.1365-2966.2004.07187.x
2190:10.1111/j.1365-2966.2006.10919.x
1374:10.1111/j.1365-2966.2005.08894.x
862:to be 21 ± 7 parts per million.
2535:
2500:
2467:
2445:
2378:
2325:
2265:Burns, Jack O. (January 2021).
2258:
2205:
2037:
1982:
1941:
1906:
1847:
1806:
1768:
1741:
1705:
1675:
1648:
1603:
1554:
865:
675:, so most broadening is due to
198:. It is frequently observed in
3036:(main website). Archived from
2510:Civilized Life in the Universe
1748:Wynn-Williams, Gareth (1992).
1516:
1464:Reports on Progress in Physics
1389:
1330:
1281:
1204:
1158:
1135:
667:, its long lifetime gives the
1:
3021:— PAST experiment description
2810:10.1103/PhysRevLett.92.211301
2129:10.1088/1475-7516/2013/11/066
1494:10.1088/0034-4885/75/8/086901
1152:
725:could produce radiation at a
718:who, in 1944, predicted that
2558:
2046:Astronomy & Astrophysics
1128:
682:
288:discretization of the total
7:
2480:Frontiers in Human Dynamics
2076:10.1051/0004-6361/201014707
1401:"The Atomic Hydrogen Maser"
1253:American Journal of Physics
1214:American Journal of Physics
1080:
173:constant of proportionality
10:
3198:
2966:10.1088/0004-637X/763/2/90
2421:10.1007/s10686-021-09743-7
1935:10.1103/PhysRevLett.21.775
1093:Chronology of the universe
639:This transition is highly
257:
89:producing this line has a
18:
2935:The Astrophysical Journal
2885:The Astrophysical Journal
2836:The Astrophysical Journal
2671:The Astrophysical Journal
2620:The Astrophysical Journal
2569:The Astrophysical Journal
2493:10.3389/fhumd.2021.714616
2334:The Astrophysical Journal
2214:The Astrophysical Journal
1428:10.1088/0026-1394/1/1/004
1141:The "I" in H I is a
301:In the ground state, the
87:electromagnetic radiation
2507:Basalla, George (2006).
1995:The Astronomical Journal
1458:Pritchard, Jonathan R.;
1337:Nusser, Adi (May 2005).
1192:10.1109/TIM.1970.4313902
1122:Timeline of the Big Bang
1103:Hydrogen spectral series
1098:Dark Ages Radio Explorer
253:
196:electromagnetic spectrum
135:Planck–Einstein relation
21:hydrogen spectral series
2779:Physical Review Letters
2515:Oxford University Press
2486:. Frontiers Media S.A.
2456:. The Planetary Society
2068:2011A&A...525A.128P
1915:Physical Review Letters
1792:1969ARA&A...7...39K
976:and American physicist
850:atoms by measuring the
828:fine-structure constant
771:
278:magnetic dipole moments
236:fine-structure constant
71:change between the two
42:21 centimeter line
2391:Experimental Astronomy
2301:10.1098/rsta.2019.0564
932:
698:
577:is the photon energy,
565:of an emitted photon,
548:
313:. When applied to the
63:. It is produced by a
33:
3162:Emission spectroscopy
1751:The Fullness of Space
930:
896:matter power spectrum
892:mapping the intensity
690:
665:uncertainty principle
549:
258:Further information:
31:
1819:Astronomical Journal
908:synchrotron emission
716:Hendrik van de Hulst
704:centre of the Galaxy
324:
303:spin-flip transition
58:electrically neutral
2958:2013ApJ...763...90W
2908:2005ApJ...626....1B
2859:2005ApJ...625..575S
2802:2004PhRvL..92u1301L
2749:2004MNRAS.347..187F
2694:2004ApJ...608..622Z
2643:2003ApJ...596....1C
2592:1997ApJ...475..429M
2413:2021ExA....51.1641K
2356:2006ApJ...650..529W
2293:2021RSPTA.37990564B
2236:2005ApJ...633..552K
2181:2006MNRAS.372..679M
2121:2013JCAP...11..066F
2007:2007AJ....133.1625R
1975:10.1093/ptep/ptu142
1966:2014PTEP.2014k1E01S
1927:1968PhRvL..21..775V
1882:1998MNRAS.295..457D
1831:1969AJ.....74..859R
1713:Bowditch, Nathaniel
1699:1957BAN....13..201W
1669:1954BAN....12..117V
1626:1951Natur.168..357M
1581:1951Natur.168..356E
1486:2012RPPh...75h6901P
1420:1965Metro...1....7R
1365:2005MNRAS.359..183N
1308:2009JPCRD..38..565W
1227:1982AmJPh..50..698G
1184:1970ITIM...19..200H
860:galactic anticenter
671:an extremely small
653:stimulated emission
260:Hyperfine structure
133:. According to the
3172:Physical cosmology
3026:"LOFAR experiment"
1178:. IM-19 (4): 200.
939:, attached to the
933:
912:free–free emission
836:interstellar space
777:In radio astronomy
758:Harvard University
699:
544:
286:quantum mechanical
280:ascribed to their
179:, is known as the
34:
2544:"Смотри в корень"
2528:978-0-19-517181-5
1717:"10. Radio Waves"
1620:(4270): 357–358.
1397:Ramsey, Norman F.
1316:10.1063/1.3077727
1275:10.1119/5.0065381
1071:irrational number
534:
520:
484:
455:
445:
415:
400:
360:
341:
3189:
3157:Hydrogen physics
3139:
3138:
3137:
3127:
3126:
3125:
3115:
3114:
3113:
3103:
3102:
3091:
3090:
3089:
3079:
3078:
3077:
3067:
3066:
3058:
3049:
3041:
3029:
3020:
3018:
3016:astro-ph/0404083
3005:
2985:
2951:
2927:
2901:
2899:astro-ph/0410129
2878:
2852:
2850:astro-ph/0408515
2829:
2795:
2793:astro-ph/0312134
2770:
2760:
2742:
2740:astro-ph/0305065
2713:
2687:
2685:astro-ph/0311514
2662:
2636:
2634:astro-ph/0303249
2611:
2585:
2583:astro-ph/9608010
2548:
2547:
2539:
2533:
2532:
2504:
2498:
2497:
2495:
2471:
2465:
2464:
2462:
2461:
2449:
2443:
2442:
2432:
2406:
2397:(3): 1641–1676.
2382:
2376:
2375:
2349:
2347:astro-ph/0501081
2329:
2323:
2322:
2312:
2286:
2262:
2256:
2255:
2229:
2227:astro-ph/0501086
2209:
2203:
2202:
2192:
2174:
2172:astro-ph/0603518
2150:
2141:
2140:
2114:
2094:
2088:
2087:
2061:
2041:
2035:
2034:
2001:(4): 1625–1632.
1986:
1980:
1979:
1977:
1945:
1939:
1938:
1910:
1904:
1903:
1893:
1875:
1873:astro-ph/9711290
1851:
1845:
1844:
1842:
1810:
1804:
1803:
1772:
1766:
1765:
1745:
1739:
1738:
1732:
1731:
1721:
1709:
1703:
1702:
1679:
1673:
1672:
1652:
1646:
1645:
1634:10.1038/168357a0
1607:
1601:
1600:
1589:10.1038/168356a0
1558:
1552:
1551:
1540:10.2307/20023736
1520:
1514:
1513:
1479:
1455:
1446:
1445:
1443:
1442:
1405:
1399:(January 1965).
1393:
1387:
1386:
1376:
1358:
1356:astro-ph/0409640
1334:
1328:
1327:
1285:
1279:
1278:
1268:
1248:
1239:
1238:
1208:
1202:
1201:
1199:
1198:
1171:
1162:
1146:
1139:
1067:
1059:
1057:
1054:
1048:
1046:
1043:
1039:
1032:
1020:
1018:
1015:
1009:
1007:
1004:
1000:
994:
991:
983:Pyotr Makovetsky
974:Giuseppe Cocconi
886:. Including the
821:
795:microwave window
736:
734:
650:
648:
632:
630:
627:
624:
613:
611:
608:
605:
588:
580:
576:
568:
560:
553:
551:
550:
545:
542:
532:
525:
518:
507:
506:
498:
489:
482:
481:
480:
456:
454:
453:
443:
442:
441:
418:
416:
413:
408:
398:
397:
396:
372:
361:
353:
342:
334:
312:
310:
290:angular momentum
226:and his student
178:
170:
165:
162:
159:
145:
128:
126:
123:
120:
110:
105:
102:
98:
75:of the hydrogen
73:hyperfine levels
3197:
3196:
3192:
3191:
3190:
3188:
3187:
3186:
3167:Radio astronomy
3147:
3146:
3145:
3135:
3133:
3123:
3121:
3111:
3109:
3097:
3087:
3085:
3075:
3073:
3061:
3053:
3048:(main website).
3044:
3032:
3028:(main website).
3024:
2996:
2993:
2988:
2561:
2556:
2554:Further reading
2551:
2542:Makovetsky, P.
2540:
2536:
2529:
2505:
2501:
2472:
2468:
2459:
2457:
2450:
2446:
2383:
2379:
2330:
2326:
2263:
2259:
2210:
2206:
2151:
2144:
2095:
2091:
2042:
2038:
1987:
1983:
1946:
1942:
1921:(11): 775–778.
1911:
1907:
1852:
1848:
1811:
1807:
1773:
1769:
1762:
1746:
1742:
1729:
1727:
1719:
1710:
1706:
1680:
1676:
1653:
1649:
1608:
1604:
1559:
1555:
1521:
1517:
1456:
1449:
1440:
1438:
1403:
1394:
1390:
1335:
1331:
1286:
1282:
1259:(12): 948–960.
1249:
1242:
1235:10.1119/1.12733
1209:
1205:
1196:
1194:
1169:
1163:
1159:
1155:
1150:
1149:
1140:
1136:
1131:
1126:
1117:Rydberg formula
1083:
1065:
1055:
1052:
1050:
1044:
1041:
1037:
1035:
1030:
1016:
1013:
1011:
1005:
1002:
998:
996:
992:
989:
978:Philip Morrison
925:
868:
844:G. L. Verschuur
819:
812:circular motion
779:
774:
732:
730:
685:
661:hydrogen clouds
646:
644:
631:.768(2) Hz
628:
625:
622:
620:
609:
606:
603:
601:
586:
583:Planck constant
578:
574:
566:
558:
535:
521:
499:
494:
493:
485:
476:
472:
446:
434:
430:
419:
412:
401:
389:
385:
373:
371:
352:
333:
325:
322:
321:
315:Planck relation
308:
306:
262:
256:
220:H. van de Hulst
200:radio astronomy
181:Planck constant
176:
163:
160:
157:
155:
137:
124:
121:
118:
116:
103:
100:
96:
94:
24:
17:
12:
11:
5:
3195:
3185:
3184:
3179:
3177:Astrochemistry
3174:
3169:
3164:
3159:
3144:
3143:
3131:
3119:
3107:
3095:
3083:
3071:
3051:
3050:
3042:
3040:on 2005-02-11.
3030:
3022:
3006:
2992:
2991:External links
2989:
2987:
2986:
2928:
2916:10.1086/429954
2879:
2867:10.1086/429857
2843:(2): 575–587.
2830:
2786:(21): 211301.
2772:
2771:
2733:(1): 187–195.
2715:
2714:
2702:10.1086/386327
2678:(2): 622–635.
2664:
2663:
2651:10.1086/377634
2613:
2612:
2600:10.1086/303549
2576:(2): 429–444.
2562:
2560:
2557:
2555:
2552:
2550:
2549:
2534:
2527:
2499:
2466:
2444:
2377:
2364:10.1086/506597
2340:(2): 529–537.
2324:
2257:
2244:10.1086/444370
2220:(2): 552–559.
2204:
2165:(2): 679–692.
2142:
2089:
2036:
2015:10.1086/511978
1981:
1960:(11): 111E01.
1940:
1905:
1866:(2): 457–462.
1846:
1840:10.1086/110874
1805:
1776:Kerr, Frank J.
1767:
1760:
1740:
1704:
1683:Westerhout, G.
1674:
1647:
1602:
1553:
1534:(7): 347–349.
1524:Purcell, E. M.
1515:
1447:
1388:
1349:(1): 183–190.
1329:
1302:(3): 565–720.
1280:
1240:
1221:(8): 698–703.
1203:
1156:
1154:
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1079:
1061:
1060:
1022:
1021:
937:Pioneer plaque
924:
921:
867:
864:
832:magnetic field
816:rotation curve
783:radio spectrum
778:
775:
773:
770:
735:.4058 MHz
712:emission lines
684:
681:
677:Doppler shifts
657:hydrogen maser
634:
633:
614:
591:speed of light
555:
554:
541:
538:
531:
528:
524:
517:
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370:
367:
364:
359:
356:
351:
348:
345:
340:
337:
332:
329:
317:, this gives:
255:
252:
248:hydrogen maser
202:because those
194:region of the
61:hydrogen atoms
15:
9:
6:
4:
3:
2:
3194:
3183:
3180:
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2516:
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2503:
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2431:
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2422:
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2400:
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2381:
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2369:
2365:
2361:
2357:
2353:
2348:
2343:
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2335:
2328:
2320:
2316:
2311:
2306:
2302:
2298:
2294:
2290:
2285:
2280:
2276:
2272:
2268:
2261:
2253:
2249:
2245:
2241:
2237:
2233:
2228:
2223:
2219:
2215:
2208:
2200:
2196:
2191:
2186:
2182:
2178:
2173:
2168:
2164:
2160:
2156:
2149:
2147:
2138:
2134:
2130:
2126:
2122:
2118:
2113:
2108:
2104:
2100:
2093:
2085:
2081:
2077:
2073:
2069:
2065:
2060:
2055:
2051:
2047:
2040:
2032:
2028:
2024:
2020:
2016:
2012:
2008:
2004:
2000:
1996:
1992:
1985:
1976:
1971:
1967:
1963:
1959:
1955:
1951:
1944:
1936:
1932:
1928:
1924:
1920:
1916:
1909:
1901:
1897:
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1874:
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1832:
1828:
1824:
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1801:
1797:
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1789:
1785:
1781:
1777:
1771:
1763:
1761:9780521426381
1757:
1753:
1752:
1744:
1737:
1725:
1718:
1714:
1708:
1700:
1696:
1692:
1688:
1684:
1678:
1670:
1666:
1662:
1658:
1651:
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1623:
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1615:
1614:
1606:
1598:
1594:
1590:
1586:
1582:
1578:
1575:(4270): 356.
1574:
1570:
1569:
1564:
1563:Purcell, E.M.
1557:
1549:
1545:
1541:
1537:
1533:
1529:
1525:
1519:
1511:
1507:
1503:
1499:
1495:
1491:
1487:
1483:
1478:
1473:
1470:(8): 086901.
1469:
1465:
1461:
1460:Loeb, Abraham
1454:
1452:
1437:
1433:
1429:
1425:
1421:
1417:
1413:
1409:
1402:
1398:
1392:
1384:
1380:
1375:
1370:
1366:
1362:
1357:
1352:
1348:
1344:
1340:
1333:
1325:
1321:
1317:
1313:
1309:
1305:
1301:
1297:
1296:
1291:
1284:
1276:
1272:
1267:
1262:
1258:
1254:
1247:
1245:
1236:
1232:
1228:
1224:
1220:
1216:
1215:
1207:
1193:
1189:
1185:
1181:
1177:
1176:
1168:
1161:
1157:
1144:
1143:roman numeral
1138:
1134:
1123:
1120:
1118:
1115:
1113:
1109:
1106:
1104:
1101:
1099:
1096:
1094:
1091:
1089:
1088:Balmer series
1086:
1085:
1078:
1076:
1072:
1068:
1033:
1027:
1026:
1025:
988:
987:
986:
984:
979:
975:
971:
966:
964:
959:
955:
951:
946:
942:
938:
929:
920:
918:
913:
909:
905:
900:
897:
893:
889:
885:
881:
880:recombination
877:
873:
863:
861:
855:
853:
849:
845:
841:
840:Zeeman effect
837:
833:
829:
824:
817:
813:
809:
808:visible light
805:
801:
796:
792:
788:
784:
769:
767:
763:
759:
755:
751:
746:
744:
743:hydrogen atom
740:
728:
724:
721:
717:
713:
709:
705:
697:
693:
689:
680:
678:
674:
673:natural width
670:
669:spectral line
666:
662:
658:
654:
642:
637:
636:in a vacuum.
618:
615:
599:
596:
595:
594:
592:
584:
572:
564:
529:
526:
515:
511:
508:
503:
500:
490:
477:
473:
469:
466:
462:
458:
438:
435:
431:
427:
424:
420:
409:
393:
390:
386:
382:
379:
375:
368:
365:
362:
357:
354:
349:
346:
343:
338:
335:
330:
327:
320:
319:
318:
316:
304:
299:
296:
291:
287:
283:
279:
275:
271:
267:
261:
251:
249:
245:
241:
237:
233:
229:
225:
224:E. M. Purcell
221:
217:
216:visible light
213:
209:
205:
201:
197:
193:
189:
184:
182:
174:
169:
153:
149:
144:
140:
136:
132:
114:
109:
92:
88:
84:
81:
78:
74:
70:
69:quantum state
66:
62:
59:
55:
51:
50:spectral line
47:
46:H I line
43:
39:
38:hydrogen line
30:
26:
22:
3141:Solar System
3038:the original
2939:
2933:
2889:
2883:
2840:
2834:
2783:
2777:
2730:
2724:
2675:
2669:
2624:
2618:
2573:
2567:
2537:
2509:
2502:
2483:
2479:
2469:
2458:. Retrieved
2447:
2394:
2390:
2380:
2337:
2333:
2327:
2274:
2270:
2260:
2217:
2213:
2207:
2162:
2158:
2102:
2098:
2092:
2049:
2045:
2039:
1998:
1994:
1984:
1957:
1953:
1943:
1918:
1914:
1908:
1863:
1859:
1849:
1822:
1818:
1808:
1783:
1779:
1770:
1750:
1743:
1734:
1728:. Retrieved
1723:
1707:
1690:
1686:
1677:
1660:
1656:
1650:
1617:
1611:
1605:
1572:
1566:
1561:Ewen, H.I.;
1556:
1531:
1527:
1518:
1467:
1463:
1439:. Retrieved
1411:
1407:
1391:
1346:
1342:
1332:
1299:
1293:
1283:
1256:
1252:
1218:
1212:
1206:
1195:. Retrieved
1173:
1160:
1137:
1062:
1023:
967:
963:Solar System
934:
901:
884:reionization
869:
866:In cosmology
856:
852:polarization
848:antihydrogen
834:strength of
825:
780:
762:Christiansen
747:
739:ground state
700:
692:Horn antenna
638:
635:
616:
612:(30) cm
597:
556:
300:
295:lower energy
294:
274:ground state
263:
185:
142:
138:
127:(30) cm
83:ground state
45:
41:
37:
35:
25:
3129:Outer space
3117:Spaceflight
2892:(1): 1–11.
2517:. pp.
2321:. 20190564.
2105:(11): 066.
1825:: 859–876.
1414:(1): 7–15.
800:cosmic dust
787:L band
268:bound to a
208:cosmic dust
204:radio waves
166:(81) μ
3151:Categories
2627:(1): 1–8.
2460:2023-04-26
2404:1908.04296
2284:2003.06881
1978:. 111E016.
1730:2023-04-28
1441:2023-04-27
1408:Metrologia
1266:2208.06070
1197:2023-04-30
1153:References
1112:nanometers
945:Pioneer 11
941:Pioneer 10
904:ionosphere
820:21 cm
563:wavelength
228:H. E. Ewen
113:wavelength
3093:Astronomy
3081:Chemistry
2982:118712522
2974:0004-637X
2949:1211.6450
2942:(2): 90.
2710:119439713
2608:118239661
2559:Cosmology
2372:119595472
2137:250754168
2112:1311.4574
2059:1010.0292
2023:1538-3881
1512:. 086901.
1477:1109.6012
1436:250873158
1324:0047-2689
1129:Footnotes
1058: GHz
1047: MHz
1019: GHz
1008: MHz
954:Voyager 2
950:Voyager 1
876:dark ages
802:that are
766:Milky Way
727:frequency
696:Milky Way
683:Discovery
649:10 s
641:forbidden
571:frequency
509:≈
501:−
491:⋅
470:⋅
459:⋅
436:−
428:⋅
410:⋅
391:−
383:⋅
369:≈
363:⋅
344:⋅
339:ν
328:λ
311: μeV
240:cosmology
232:Milky Way
210:that are
192:microwave
106:(2)
91:frequency
3182:Hydrogen
2875:15464776
2826:30510359
2818:15245272
2659:10258589
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1642:32329393
1597:27595927
1548:20023736
1510:41341641
1502:22828208
1383:11547883
1081:See also
1075:harmonic
888:redshift
872:Big Bang
791:UHF band
785:(in the
723:hydrogen
708:Jan Oort
655:using a
266:electron
54:solitary
3069:Physics
3055:Portals
3004:(NRAO).
2954:Bibcode
2924:7343629
2904:Bibcode
2855:Bibcode
2798:Bibcode
2767:6474189
2745:Bibcode
2690:Bibcode
2639:Bibcode
2588:Bibcode
2519:133–135
2430:8416573
2409:Bibcode
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2310:7739898
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2232:Bibcode
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1962:Bibcode
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1180:Bibcode
1108:H-alpha
958:pulsars
878:" from
793:of the
789:of the
754:Purcell
741:of the
720:neutral
589:is the
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152:energy
148:photon
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131:vacuum
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3105:Stars
3011:arXiv
2978:S2CID
2944:arXiv
2920:S2CID
2894:arXiv
2871:S2CID
2845:arXiv
2822:S2CID
2788:arXiv
2763:S2CID
2735:arXiv
2706:S2CID
2680:arXiv
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2629:arXiv
2604:S2CID
2578:arXiv
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2248:S2CID
2222:arXiv
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2167:arXiv
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1506:S2CID
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