question: If we see galaxies light years away, how do we know we’re not seeing our own galaxy X light years in the past?
comment: We can't see light emitted from our own galaxy in the past because that would require our galaxy to have travelled faster than the light. And nothing travels faster than light. It would be like throwing a ball down the room and trying to run and catch it at the other end.
passages: Receiving light and other signals from distant astronomical sources can even take much longer. For example, it has taken 13 billion (13) years for light to travel to Earth from the faraway galaxies viewed in the Hubble Ultra Deep Field images. Those photographs, taken today, capture images of the galaxies as they appeared 13 billion years ago, when the universe was less than a billion years old. The fact that more distant objects appear to be younger, due to the finite speed of light, allows astronomers to infer the evolution of stars, of galaxies, and of the universe itself.
   The "distance" of a far away galaxy depends on what distance measurement you use. With a redshift of 5.34, light from this galaxy is estimated to have taken around 12.5 billion years to reach us. But since this galaxy is receding from Earth, the present comoving distance is estimated to be around 26 billion light-years.
   The figures quoted above are distances "now" (in cosmological time), not distances "at the time the light was emitted". For example, the cosmic microwave background radiation that we see right now was emitted at the time of photon decoupling, estimated to have occurred about years after the Big Bang, which occurred around 13.8 billion years ago. This radiation was emitted by matter that has, in the intervening time, mostly condensed into galaxies, and those galaxies are now calculated to be about 46 billion light-years from us. To estimate the distance to that matter at the time the light was emitted, we may first note that according to the Friedmann–Lemaître–Robertson–Walker metric, which is used to model the expanding universe, if at the present time we receive light with a redshift of "z", then the scale factor at the time the light was originally emitted is given by
   Though in principle more galaxies will become observable in the future, in practice an increasing number of galaxies will become extremely redshifted due to ongoing expansion, so much so that they will seem to disappear from view and become invisible. An additional subtlety is that a galaxy at a given comoving distance is defined to lie within the "observable universe" if we can receive signals emitted by the galaxy at any age in its past history (say, a signal sent from the galaxy only 500 million years after the Big Bang), but because of the universe's expansion, there may be some later age at which a signal sent from the same galaxy can "never" reach the Earth at any point in the infinite future (so, for example, we might never see what the galaxy looked like 10 billion years after the Big Bang), even though it remains at the same comoving distance (comoving distance is defined to be constant with time—unlike proper distance, which is used to define recession velocity due to the expansion of space), which is less than the comoving radius of the observable universe. This fact can be used to define a type of cosmic event horizon whose distance from the Earth changes over time. For example, the current distance to this horizon is about 16 billion light-years, meaning that a signal from an event happening "at present" can eventually reach the Earth in the future if the event is less than 16 billion light-years away, but the signal will never reach the Earth if the event is more than 16 billion light-years away.
   BULLET::::- Elsewhere is the region between the two light cones. Points in an observer's elsewhere are inaccessible to her/him; only points in the past can send signals to the observer. In ordinary laboratory experience, using common units and methods of measurement, it may seem that we look at the present, but in fact there is always a delay time for light to propagate. For example, we see the Sun as it was about 8 minutes ago, not as it is "right now". Unlike the present in Galilean/Newtonian theory, the elsewhere is thick; it is not a 3-dimensional volume but is instead a 4-dimensional spacetime region.
   One possible explanation for the missing light-source is that illumination from the assumed quasar was a transient phenomenon. In this case, its effects on HsV would be still visible because of the distance of several tens of thousands of light years between HsV and the quasar in the nearby galaxy: HsV would show a "light echo" or "ghost image" of events that are older than those currently seen in the galaxy.
   The more distant an object is, the younger it appears: its light has taken longer to reach human observers. Because the universe is expanding, as the light travels it becomes red-shifted, and objects at extreme distances are therefore easier to see if viewed in the infrared. JWST's infrared capabilities are expected to let it see back in time to the first galaxies forming just a few hundred million years after the Big Bang.
   On 25 June 2018, astronomers, using the Liverpool Telescope and the telescope at Palomar Observatory, noted on "The Astronomer's Telegram": "AT2018cow has faded every night since our first observations . ... observations suggest that although a link to Ic-BL SNe and GRBs remains credible given the smooth spectra and luminous radio and X-ray counterpart, AT2018cow is distinct in other ways and its true identity remains unclear. Observations are continuing." On 29 June 2018, astronomers, using telescopes at the Beijing Astronomical Observatory, and on 30 June 2018, using the Swift/XRT telescope, reported further support for the fading of AT2018cow.
   The stars are very far away from us, so we only observe their one characteristic feature, their light. When this light is passed through a prism, it gives rise to a spectrum. Every star has its own spectrum and since each element has its own unique spectra, we can know a star's composition. We use thermal spectra of the stars to know their temperature. In 1920, when scientists were examining spectra of different stars, they found that some of the characteristic lines of the star spectrum was shifted towards the red end of the spectrum. The implications of this phenomenon was given by the Doppler effect, and it was clear that some stars were moving away from us. 
   BULLET::::- 1959 — Hundreds of radio sources are detected by the Cambridge Interferometer which produces the 3C catalogue. Many of these are later found to be distant quasars and radio galaxies
  
  BULLET::::- 1960 — Thomas Matthews determines the radio position of the "3C" source 3C 48 to within 5",
  
  BULLET::::- 1960 — Allan Sandage optically studies 3C 48 and observes an unusual blue quasistellar object,
  
  BULLET::::- 1962 — Cyril Hazard, M. B. Mackey, and A. J. Shimmins use lunar occultations to determine a precise position for the quasar 3C 273 and deduce that it is a double source,
   However, observation and discovery of extragalactic supernovae are now far more common. The first such observation was of SN 1885A in the Andromeda Galaxy. Today, amateur and professional astronomers are finding several hundred every year, some when near maximum brightness, others on old astronomical photographs or plates. American astronomers Rudolph Minkowski and Fritz Zwicky developed the modern supernova classification scheme beginning in 1941. During the 1960s, astronomers found that the maximum intensities of supernovae could be used as standard candles, hence indicators of astronomical distances. Some of the most distant supernovae observed in 2003, appeared dimmer than expected. This supports the view that the expansion of the universe is accelerating. Techniques were developed for reconstructing supernovae events that have no written records of being observed. The date of the Cassiopeia A supernova event was determined from light echoes off nebulae, while the age of supernova remnant RX J0852.0-4622 was estimated from temperature measurements and the gamma ray emissions from the radioactive decay of titanium-44.
   Tyson begins the episode by explaining the nature of the speed of light and how much of what is seen of the observable universe is from light emanated from billions of years ago. Tyson further explains how modern astronomy has used such analysis via deep time to identify the Big Bang event and the age of the universe.
   BULLET::::- 1989 — Margaret Geller and John Huchra discover the "Great Wall", a sheet of galaxies more than 500 million light years long and 200 million wide, but only 15 million light years thick,
  
  BULLET::::- 1990 — Michael Rowan-Robinson and Tom Broadhurst discover that the IRAS galaxy IRAS F10214+4724 is the brightest known object in the Universe,
   In October 2018, astronomers reported that GRB 150101B, 1.7 billion light years away from Earth, may be analogous to the historic GW170817. It was detected on 1 January 2015 at 15:23:35 UT by the Gamma-ray Burst Monitor on board the Fermi Gamma-ray Space Telescope, along with detections by the Burst Alert Telescope (BAT) on board the Swift Observatory Satellite.
   Section::::Discovery.
  
  The Baby Boom Galaxy was discovered and characterized in 2008 using a suite of telescopes operating at different wavelengths. NASA's Hubble Space Telescope and Japan's Subaru Telescope, atop Mauna Kea in Hawaii, first spotted the galaxy in visible-light images, where it appeared as an inconspicuous smudge due to its great distance. It wasn't until the Spitzer and the James Clerk Maxwell Telescope, also on Mauna Kea in Hawaii, observed the galaxy at infrared and submillimeter wavelengths, respectively, that the galaxy was formally discovered.
  
  Section::::Strange behavior.
   The proper distance—the distance as would be measured at a specific time, including the present—between Earth and the edge of the observable universe is 46 billion light-years (14 billion parsecs), making the diameter of the observable universe about 93 billion light-years (28 billion parsecs). The distance the light from the edge of the observable universe has travelled is very close to the age of the Universe times the speed of light, , but this does not represent the distance at any given time because the edge of the observable universe and the Earth have since moved further apart. For comparison, the diameter of a typical galaxy is 30,000 light-years (9,198 parsecs), and the typical distance between two neighboring galaxies is 3 million light-years (919.8 kiloparsecs). As an example, the Milky Way is roughly 100,000–180,000 light-years in diameter, and the nearest sister galaxy to the Milky Way, the Andromeda Galaxy, is located roughly 2.5 million light-years away.
   The "distance" of a far away galaxy depends on what distance measurement you use. With a redshift of 5.47, light from this active galaxy is estimated to have taken around 12.3 billion light-years to reach us. But since this galaxy is receding from Earth at an estimated rate of 285,803 km/s (the speed of light is 299,792 km/s), the present (co-moving) distance to this galaxy is estimated to be around 26 billion light-years (7961 Mpc).
  
  Section::::Statistics.
  
  BULLET::::- Epoch 2000.0
  
  BULLET::::- RA: 09h 06m 30.8s
  
  BULLET::::- Dec: +69° 30' 31"
  
  BULLET::::- Classification: FSRQ
  
  BULLET::::- Redshift (z) = 5.47
  
  BULLET::::- R = 19.9
   Because the source was nonrepeating, the astronomers had to scan large areas with the 36 telescopes of ASKAP. Once a signal was found, they used the Very Large Telescope, the Gemini Observatory in Chile, and the W. M. Keck Observatory in Hawaii to identify its host galaxy and determine its distance. Knowing the distance and source galaxy properties, enables a study of the composition of the intergalactic medium.
  
  Section::::Bursts observed.:2019.:June 2019 Report.
   The galaxy was identified in the Hubble telescope Reionization Lensing Cluster Survey (RELICS) and companion S-RELICS Spitzer program and is at the limits of Hubble's detection capabilities. As a consequence of the effect of a gravitational field of a galaxy cluster of an extremely large size, SPT-CL J0615-5746, (abbreviated to SPT0615), situated at a distance closer to earth, light from SPT0615-JD located at a further distance, is amplified and distorted (lensed - Einstein 1936; Khvolson 1924; Link 1936) on it's motion to the Hubble telescope. This distortion causes the light from the galaxy to arrive as an image lengthened to an arc of about 2 arcseconds long. 
   (i) Forward evolution, which begins with cosmological initial conditions and follows a forward evolution with time by means of semi-analytical models of galaxy formation.
  
  (ii) Backward evolution, which begins with existing galaxy populations and extrapolates them backwards in time.
  
  (iii) Evolution of the galaxy populations that is inferred over a range of redshifts. The galaxy evolution is inferred here using some quantity derived from observations such as the star formation rate density of the universe.
  
  (iv) Evolution of the galaxy populations that is directly observed over the range of redshifts that contribute significantly to the EBL.
  
  Section::::See also.
   According to this approach, the relation "cz" = "v" is an approximation valid at low redshifts, to be replaced by a relation at large redshifts that is model-dependent. See velocity-redshift figure.
  
  Section::::Interpretation.:Observability of parameters.
  
  Strictly speaking, neither "v" nor "D" in the formula are directly observable, because they are properties "now" of a galaxy, whereas our observations refer to the galaxy in the past, at the time that the light we currently see left it.
   When one looks out into the night sky, distances also correspond to time into the past. A galaxy measured at ten billion light years in distance appears to us as it was ten billion years ago, because the light has taken that long to travel to the observer. If one were to look at a galaxy ten billion light years away in one direction and another in the opposite direction, the total distance between them is twenty billion light years. This means that the light from the first has not yet reached the second, because the approximately 13.8 billion years that the universe has existed is not a long enough time to allow it to occur. In a more general sense, there are portions of the universe that are visible to us, but invisible to each other, outside each other's respective particle horizons.
   Section::::Operations.
  
  Operational from 1996–1997, MSX mapped the galactic plane and areas either missed or identified as particularly bright by the Infrared Astronomical Satellite (IRAS) at wavelengths of 4.29 µm, 4.35 µm, 8.28 µm, 12.13 µm, 14.65 µm, and 21.3 µm. It carried the 33-cm SPIRIT III infrared telescope with solid hydrogen-cooled five line-scanned infrared focal plane arrays.
   Since the 1960s, telescopes and astronomical instrumentation have advanced greatly: the Hubble Space Telescope was launched, and cosmological theory and observation has advanced considerably. Black holes and supermassive black holes have been directly as well as indirectly detected, extremely distant objects are routinely studied and contextualized, and multiple 8-10 meter telescopes (such as those at Keck Observatory and the Very Large Telescope) have become operational, and detectors such as CCDs are now more widely employed. These developments, have led to quasars being understood to be very distant active galaxies with high redshifts. Many imaging surveys, most notably the Hubble Deep Field, have found many high-redshift objects that are not QSOs but that appear to be normal galaxies like those found nearby. The spectra of the high-redshift galaxies, as seen from X-ray to radio wavelengths, match the spectra of nearby galaxies (particularly galaxies with high levels of star formation activity but also galaxies with normal or extinguished star formation activity) when corrected for redshift effects.
   Starlight also follows the inverse square law. Light rays leave the star in equal amounts in all directions. The light rays create a sphere of light surrounding the star. As time progresses, the sphere grows as the light rays travel through space away from the star. While the sphere of light grows, the number of light rays stays the same. So, the amount of light per unit of surface area of the sphere (called flux in astronomy) decreases with distance and therefore time. When observing a star, only the light rays that are in the given area being viewed can be detected. This is why a star appears dimmer the farther away it is.
  