explain xkcd - User contributions [en]

Talk:2853: Redshift

2023-11-10T21:16:09Z

Juandesant:

Does it make any sense to try and relate the two Z values to a more "normal" time? [[User:MAP|MAP]] ([[User talk:MAP|talk]]) 21:08, 10 November 2023 (UTC)

I have added exactly that, using `astropy.cosmology` for the calculations [[User:Juandesant|Juandesant]] ([[User talk:Juandesant|talk]]) 21:16, 10 November 2023 (UTC)

2853: Redshift

2023-11-10T21:14:34Z

Juandesant: /* Explanation */

{{comic
| number = 2853
| date = November 10, 2023
| title = Redshift
| image = redshift_2x.png
| imagesize = 223x264px
| noexpand = true
| titletext = So do you have any plans for z=-0.000000000000045?
}}

==Explanation==
{{incomplete|Created by a SHIFTY RED MATTER BALL - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

In cosmology, redshift refers to the way that light from distant objects in the universe is stretched out, making it appear more red than it would otherwise. This occurs because the universe is expanding, and as a result, light waves are stretched as they travel through space. The "z" value is a dimensionless measure of the redshift. A higher "z" value, or redshift, corresponds to earlier times in the history of the universe. This is because as the universe expands, light from distant galaxies is stretched to longer, redder wavelengths as it travels towards us. The further away a galaxy is, the longer its light has been traveling, and thus the more the universe has expanded since that light began its journey. Therefore, a higher redshift indicates a galaxy that is further away and that the light we see from it left when the universe was younger. Conversely, a lower redshift means the light has traveled a shorter distance and time, indicating a more recent epoch in the history of the universe. Negative values of "z" indicate a blueshift, which indicate objects that are approaching the observer, generally used in cosmological work to calculate rotation speeds of nearby objects.

The joke here is that Cueball is asking another when they became interested in cosmology, and instead of giving a time like "in college" or "as a kid," they respond with a very precise redshift value "z=0.00000000038." This absurdly precise number is due to a highly recent event compared to the start of the universe, within the cosmologist's lifetime. The negative blueshift question in the title text is a playful way of asking about a future event, as it is approaching the speakers. Specifically it is a closer event to present day to when the cosmologist became interested in her field of study as the absolute value of z is 10^4 smaller and indicates a closer albeit future event.

Assuming a particular cosmology, and in particular values for the Hubble constant (H₀) and the curvature of the universe, it is possible to specify a particular look-back time. For z = 0.00000000038, and a flat Lambda CDM cosmology, with H₀ = 69.32 km / (Mpc s), a value of Ω₀ of 0.2865, a cosmic background temperature of 2.725 K, the look-back time is of around 1960 days, or five and a half years, what would suggest that the person started studying cosmology as part of their Ph.D. program.

Negative numbers of z would indicate a "look-forward" time, or a time in the future, and the same model indicates that z = -0.000000000000045 corresponds to about 5.5 hours in the future.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:Ponytail and Cueball are sitting at a table, eating dinner.
:Cueball: So, when did you first get interested in early universe cosmology?
:Ponytail: Sometime around Z = 0.00000000038

{{comic discussion}}

2853: Redshift

2023-11-10T21:13:26Z

Juandesant: /* Explanation */ Evaluation of the lookback and lookforward times from the indicated z indexes

{{comic
| number = 2853
| date = November 10, 2023
| title = Redshift
| image = redshift_2x.png
| imagesize = 223x264px
| noexpand = true
| titletext = So do you have any plans for z=-0.000000000000045?
}}

==Explanation==
{{incomplete|Created by a SHIFTY RED MATTER BALL - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

In cosmology, redshift refers to the way that light from distant objects in the universe is stretched out, making it appear more red than it would otherwise. This occurs because the universe is expanding, and as a result, light waves are stretched as they travel through space. The "z" value is a dimensionless measure of the redshift. A higher "z" value, or redshift, corresponds to earlier times in the history of the universe. This is because as the universe expands, light from distant galaxies is stretched to longer, redder wavelengths as it travels towards us. The further away a galaxy is, the longer its light has been traveling, and thus the more the universe has expanded since that light began its journey. Therefore, a higher redshift indicates a galaxy that is further away and that the light we see from it left when the universe was younger. Conversely, a lower redshift means the light has traveled a shorter distance and time, indicating a more recent epoch in the history of the universe. Negative values of "z" indicate a blueshift, which indicate objects that are approaching the observer, generally used in cosmological work to calculate rotation speeds of nearby objects.

The joke here is that Cueball is asking another when they became interested in cosmology, and instead of giving a time like "in college" or "as a kid," they respond with a very precise redshift value "z=0.00000000038." This absurdly precise number is due to a highly recent event compared to the start of the universe, within the cosmologist's lifetime. The negative blueshift question in the title text is a playful way of asking about a future event, as it is approaching the speakers. Specifically it is a closer event to present day to when the cosmologist became interested in her field of study as the absolute value of z is 10^4 smaller and indicates a closer albeit future event.

Assuming a particular cosmology, and in particular values for the Hubble constant ($H_0$) and the curvature of the universe, it is possible to specify a particular look-back time. For z = 0.00000000038, and a flat Lambda CDM cosmology, with $H_0$ = 69.32 km / (Mpc s), a value of $\Omega_0$ of 0.2865, a cosmic background temperature of 2.725 K, the look-back time is of around 1960 days, or five and a half years, what would suggest that the person started studying cosmology as part of their Ph.D. program.

Negative numbers of z would indicate a "look-forward" time, or a time in the future, and the same model indicates that z = -0.000000000000045 corresponds to about 5.5 hours in the future.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:Ponytail and Cueball are sitting at a table, eating dinner.
:Cueball: So, when did you first get interested in early universe cosmology?
:Ponytail: Sometime around Z = 0.00000000038

{{comic discussion}}