# Accelerating universe explained by constant expansion?

• I
• RussB

#### RussB

Why is the accelerating expansion of the universe not explained simply by the perspective of acceleration caused by a constant (non accelerating) universal expansion? Let's take two particles (X,Y) in a 1d expanding universe where it expands universally by 1 unit per per unit time. O represents empty space.

T=0: XY
T=1: XOY
T=2: XOOOY
T=3: XOOOOOOOOOY

Even though the universe is expanding in a completely constant way, any given particle would perceive every other particle (not bound by some sufficiently strong force) to be accelerating away.

What you are describing is constant expansion rate, not constant expansion. Accelerating expansion means that the second derivative of the scale factor is positive. Constant expansion rate (Hubble parameter) means that the scale factor grows exponentially and hence corresponds to an accelerated expansion - as you have deduced. In a non accelerating expansion (coasting universe), the scale factor grows linearly and the Hubble parameter is decreasing with time.

## 1. What is the accelerating universe?

The accelerating universe refers to the observation that the expansion of the universe is currently speeding up, rather than slowing down as was previously thought.

## 2. What causes the constant expansion of the universe?

The constant expansion of the universe is believed to be caused by dark energy, a mysterious force that makes up about 70% of the total energy in the universe.

## 3. How does dark energy explain the accelerating universe?

Dark energy is thought to have negative pressure, which counteracts the gravitational pull of matter and causes the expansion of the universe to accelerate.

## 4. How was the constant expansion of the universe discovered?

The constant expansion of the universe was first discovered through observations of Type Ia supernovae, which are exploding stars used as "standard candles" to measure the expansion rate of the universe.

## 5. What are the implications of a constantly expanding universe?

The constant expansion of the universe has several implications, including the ultimate fate of the universe and the possibility of other universes beyond our observable universe. It also challenges our understanding of fundamental physics and the nature of dark energy.