# Cosmological constant

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mertcan
hi, I have been watching some "world science festival" videos on youtube, also there were a conversation pertain to whether or not the cosmological constant should be a constant. As far as I know, our universe is expanding with a positive acceleration measuring the red shifts, and it implies that net total energy should not be constant in a given time. Nevertheless, there was a situation that a guy in videos said that data they had acquired resembled the cosmological constant. Is not there a contradiction???????. Because they say both the our universe is expanding and data they had acquired resembled the cosmological constant (static universe, net total energy is constant).

No, there isn't a contradiction. The universe could be expanding without a cosmological constant whatsoever.
Presence of dark matter causes the expansion to accelerate, and if it's in the form of a cosmological constant, this acceleration is constant.
Models with changing dark matter result in acceleration of acceleration of the expansion.

mertcan
No, there isn't a contradiction. The universe could be expanding without a cosmological constant whatsoever.
Presence of dark matter causes the expansion to accelerate, and if it's in the form of a cosmological constant, this acceleration is constant.
Models with changing dark matter result in acceleration of acceleration of the expansion.
I would like to add that as far as I know we add cosmological constant to ensure the derivative of energy tensor equals zero. I mean, if cosmological constant (even if the dark matter result in it) is constant, then Does not the derivative of energy tensor equal zero????? ( static universe )

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Presence of dark matter causes the expansion to accelerate

No, the presence of dark energy causes acceleration. Dark matter is a regular matter component which causes deceleration. Dark matter and dark energy are very different concepts.

Because they say both the our universe is expanding and data they had acquired resembled the cosmological constant (static universe, net total energy is constant).

Einstein's introduction of the cosmological constant was to get a static universe. However, this only occurs for a very particular value for the cosmological constant and the solution is anyway unstable. There is no contradiction in having a cosmological constant and an expanding universe.

vanhees71