Editing 2764: Cosmological Nostalgia Content
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[[File:age by redshift.png|thumb|right|[[Redshift]] indicates the age of the universe for distant astronomical objects as we observe them today, when the universe is 13.78 billion years old.]] | [[File:age by redshift.png|thumb|right|[[Redshift]] indicates the age of the universe for distant astronomical objects as we observe them today, when the universe is 13.78 billion years old.]] | ||
| β | In cosmology, <i>z</i> is the symbol for {{w|Redshift|redshift}}, the effect whereby photons traveling from an object that is moving away from the observer exhibit an increase in wavelength, resulting in their color shifting towards the red end of the spectrum, and the variable <i>z</i> quantifies this amount of shift. Due to the expansion of the universe, objects that are further away from us appear to be moving away from us at a faster velocity, resulting in higher redshift. As light has finite velocity, it {{w|Speed of light#Spaceflight and astronomy|takes a longer time }}for light from faraway objects to arrive at the observer. So the light observed at the present must have been emitted by the faraway object further back in time. In this sense, after assuming a cosmological model, redshift and cosmic time can be put in a one-to-one relation and are often used interchangeably by astronomers. Redshift <i>z</i> = 10 would correspond to about 500 million years after the universe was formed, or almost 13 billion years ago[https://www.astro.ucla.edu/~wright/CosmoCalc.html]. Megan is stating that only people that were born at that time (<i>z</i> = 10) can remember when the first stars were still forming. At redshift <i>z</i> = 10 the matter filling the universe was mostly constituted of neutral hydrogen gas, referred by Megan as "cold" (it had a temperature of about 300 K) and "dark" (there were no light sources in the universe before the first stars formed. This epoch is indeed called the {{w|Timeline of the early universe#Cosmic Dark Age|cosmic dark ages}}). The ultraviolet light emitted by the first stars started ionizing the hydrogen around them in expanding hot plasma bubbles. This process (called {{w|reionization}}) had probably already begun at <i>z</i> = 10, but was really completed only at about <i>z</i> = 6, when the intergalactic hydrogen was completely ionized, as it continues to be at the present day. | + | In cosmology, <i>z</i> is the symbol for {{w|Redshift|redshift}}, the effect whereby photons traveling from an object that is moving away from the observer exhibit an increase in wavelength, resulting in their color shifting towards the red end of the spectrum, and the variable <i>z</i> quantifies this amount of shift. Due to the expansion of the universe, objects that are further away from us appear to be moving away from us at a faster velocity, resulting in higher redshift. As light has finite velocity, it {{w|Speed of light#Spaceflight and astronomy|takes a longer time}}for light from faraway objects to arrive at the observer. So the light observed at the present must have been emitted by the faraway object further back in time. In this sense, after assuming a cosmological model, redshift and cosmic time can be put in a one-to-one relation and are often used interchangeably by astronomers. Redshift <i>z</i> = 10 would correspond to about 500 million years after the universe was formed, or almost 13 billion years ago[https://www.astro.ucla.edu/~wright/CosmoCalc.html]. Megan is stating that only people that were born at that time (<i>z</i> = 10) can remember when the first stars were still forming. At redshift <i>z</i> = 10 the matter filling the universe was mostly constituted of neutral hydrogen gas, referred by Megan as "cold" (it had a temperature of about 300 K) and "dark" (there were no light sources in the universe before the first stars formed. This epoch is indeed called the {{w|Timeline of the early universe#Cosmic Dark Age|cosmic dark ages}}). The ultraviolet light emitted by the first stars started ionizing the hydrogen around them in expanding hot plasma bubbles. This process (called {{w|reionization}}) had probably already begun at <i>z</i> = 10, but was really completed only at about <i>z</i> = 6, when the intergalactic hydrogen was completely ionized, as it continues to be at the present day. |
Megan also has a red tint (she is drawn in the color [https://www.color-hex.com/color/462424 #462424], a very dark red; here is a comparison of #462424 and black: <span style="background-color: #462424"> </span> <span style="background-color: black"> </span> ), indicating that she is one such <i>z</i> = 10 kid, because her image is red-shifted, in the literal sense. The irony is that even if she were actually born 13 billion years ago (which would be absurd,{{Citation needed}}) her image would not appear red-shifted to us now. She would only appear redshifted to a far away observer looking at the young Megan wandering about the primordal cold dark gas. | Megan also has a red tint (she is drawn in the color [https://www.color-hex.com/color/462424 #462424], a very dark red; here is a comparison of #462424 and black: <span style="background-color: #462424"> </span> <span style="background-color: black"> </span> ), indicating that she is one such <i>z</i> = 10 kid, because her image is red-shifted, in the literal sense. The irony is that even if she were actually born 13 billion years ago (which would be absurd,{{Citation needed}}) her image would not appear red-shifted to us now. She would only appear redshifted to a far away observer looking at the young Megan wandering about the primordal cold dark gas. | ||
