Is Time Expanding Along with Space in the Expanding Universe?

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Discussion Overview

The discussion centers around the concept of whether time is expanding along with space in the context of the expanding universe. Participants explore the implications of this idea within the framework of spacetime, particularly in relation to general relativity and cosmological expansion.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question how we can assert that time is not expanding along with space, suggesting a need for a definition of "expansion of time."
  • One participant argues that the statement about the universe's expansion is a precise mathematical description, implying that a similar statement for time would lack meaning.
  • Another participant reflects on their initial thoughts about treating time as a coordinate that could also be "stretched" like spatial coordinates, but later realizes that this notion does not hold as time evolves.
  • It is noted that while spatial coordinates change with time due to expansion, time itself is treated differently, and changes in time can be accounted for by coordinate transformations.
  • A geometric analogy involving circles is presented to illustrate the distinction between spatial expansion and the concept of time, suggesting that time functions differently than spatial dimensions in the context of expansion.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether time can be considered to expand alongside space. There are competing views on the nature of time in relation to the expansion of the universe, and the discussion remains unresolved.

Contextual Notes

Participants express uncertainty regarding the definitions and implications of time and space in the context of cosmological expansion. The discussion highlights the complexity of relating time to spatial dimensions and the potential limitations of current understandings.

lomidrevo
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Hello all,
at the beginning, let me say that I haven't studied General relativity yet, I might have some knowledge at popsci level only... So I apologize if the question looks stupid...

At cosmological scale we observe that universe is expanding. If I understood well, this includes only the spatial coordinates of the spacetime.
How do we know that time is not expanding together with space?
 
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lomidrevo said:
Hello all,
at the beginning, let me say that I haven't studied General relativity yet, I might have some knowledge at popsci level only... So I apologize if the question looks stupid...

At cosmological scale we observe that universe is expanding. If I understood well, this includes only the spatial coordinates of the spacetime.
How do we know that time is not expanding together with space?

How would you define the expansion of time?
 
When we say that the Universe is expanding, this is the translation of a very precise mathematical statement to English for a particular way of describing it. The corresponding statement for time would be meaningless.
 
thanks guys, I needed to think about it twice to understand that such question doesn't make sense indeed

initially, my thinking was as following:
we have 4 coordinates of spacetime: x,y,z,t. During expansion x,y,z coordinates are "stretched". I imagined that time coordinate can defined as ct, in order to be measured in the same units as x,y,z. So if the x,y,z are scaled by the expansion, why couldn't be ct?

Now I realized, that whatever I understand by expansion, it must be evolving with time. So expanding time as the time evolves, makes no sense indeed.
 
lomidrevo said:
So if the x,y,z are scaled by the expansion, why couldn't be ct?

Time is different. Expansion means that the distances defined by the spatial coordinates change with time. You can try to do the same with time, but differences in time changing with time can be removed by a simple change of coordinates.
 
To take a similar example in a more familiar geometry:

Consider a set of circles in the plane. As you go to larger and larger radius, the circles become larger and larger. In the same language, we would say that the circumference of the circle expands with radius. However, it would be unclear (and not well defined what you meant) if you stated that the radius expanded with the radius (yes, you go to larger values of r, but the distance between r and r+dr is dr by definition).

In the expanding universe, time plays the role of r in this example.
 
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Orodruin said:
To take a similar example in a more familiar geometry:

Consider a set of circles in the plane. As you go to larger and larger radius, the circles become larger and larger. In the same language, we would say that the circumference of the circle expands with radius. However, it would be unclear (and not well defined what you meant) if you stated that the radius expanded with the radius (yes, you go to larger values of r, but the distance between r and r+dr is dr by definition).

In the expanding universe, time plays the role of r in this example.

Thank you for this example, it is clear now :)
 

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