With the discovery of dark energy, what best defines critical density?

a) the correct amount of matter and energy to make the universe flat.

b) the density of dark energy needed to accelerate the universe's expansion.

c) the amount of matter needed to counteract dark energy.

d) the amount of matter and energy needed to hold galaxies together

The correct answer is A. According to the Friedmann Equations, critical density is the correct amount of matter and energy to make the universe flat.

Explanation:

In cosmology, the critical density is the energy density for which space-time is flat on a large scale. In other words, the critical density is the energy density for which the spatial curvature of a homogeneous, isotropic and expanding universe is zero.

The critical density of the Universe expresses the density that discriminates the closed universe from the open one, that is to say between the one dominated by gravity (density greater than the critical one) by the one dominated by the expansion (density lower than the critical one).

magnified images possible images can be real = projected onto a screen

  distance on the y-axis and time on the x-axis.

explanation:

the instantaneous velocity = change in position/time taken

the slope of the graph should give instantaneous velocity.

  slope of a graph = change in value of y -axis / change in values of x -axis

  comparing both the equations

    change in value of y -axis = change in position

  change in values of x -axis = time taken

so position values should be on the y axis and time values should be on the x axis.

distance on the y-axis and time on the x-axis.