Editing 2764: Cosmological Nostalgia Content

The title of this comic is a combination of "{{w|cosmological constant}}" (an astrophysics term related to {{w|dark energy}} and to the accelerated {{w|expansion of the universe}}) and "[https://movementstrategy.com/editorial/nostalgia-social-media/ nostalgia content]" (clickbait marketing aimed at a specific age group referencing pop culture from their youth). The [https://tvtropes.org canonical examples] of nostalgia content are "[https://knowyourmeme.com/memes/only-90s-kids--2 Only 90s Kids Remember...]" and "[https://knowyourmeme.com/memes/feel-old-yet Feel Old yet?]", and this has also formed the basis for [[:Category:Comics to make one feel old|several XKCD comics]]. Some people of relatively advanced years like to make comparisons to others in their age group of where events that they remember fit into history; e.g., "The first moon landing was closer to the end of World War I than to today."  

The title of this comic is a combination of "{{w|cosmological constant}}" (an astrophysics term related to {{w|dark energy}} and to the accelerated {{w|expansion of the universe}}) and "[https://movementstrategy.com/editorial/nostalgia-social-media/ nostalgia content]" (clickbait marketing aimed at a specific age group referencing pop culture from their youth). The [https://tvtropes.org canonical examples] of nostalgia content are "[https://knowyourmeme.com/memes/only-90s-kids--2 Only 90s Kids Remember...]" and "[https://knowyourmeme.com/memes/feel-old-yet Feel Old yet?]", and this has also formed the basis for [[:Category:Comics to make one feel old|several XKCD comics]]. Some people of relatively advanced years like to make comparisons to others in their age group of where events that they remember fit into history; e.g., "The first moon landing was closer to the end of World War I than to today."  

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.