# 7.4 Galaxy Travel | Summary and Q&A

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November 8, 2021
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MIT OpenCourseWare
7.4 Galaxy Travel

## TL;DR

Alice, traveling at nearly the speed of light, would observe blueshifted starlight as X-rays and the cosmic microwave background as ultraviolet light during her journey towards the center of the galaxy.

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### Q: What will Alice see while looking out of the windows of the spacecraft during her journey towards the center of the galaxy?

Alice will observe blueshifted starlight as X-rays and the cosmic microwave background as ultraviolet light due to her velocity, which will make the wavelengths appear shorter.

### Q: How long does it take for Alice to accelerate from 0 to her velocity with an acceleration of 10 meters per second squared?

To calculate the time it takes for Alice to accelerate, the specific values of her spacecraft's acceleration and final velocity need to be provided. However, with an acceleration of 10 meters per second squared, she will reach her velocity in a finite amount of time.

### Q: What is the temperature of the cosmic microwave background?

The temperature of the cosmic microwave background is approximately 2.7 Kelvin, which corresponds to the spectrum of photons emitted by it.

### Q: How do fluctuations in the energy density 380,000 years after the Big Bang relate to the formation of galaxies and galaxy clusters today?

The fluctuations in energy density observed 380,000 years after the Big Bang served as seeds for the formation of galaxies and galaxy clusters as they correlate to the presence of stars and galaxies in the present.

## Summary & Key Takeaways

• Bob and Alice are on Earth, and Alice travels towards the center of the galaxy in a spacecraft moving at 0.99999998 times the speed of light.

• In Bob's frame of reference, the journey takes 30,000 years, but for Alice, it only takes two years.

• Alice will observe blueshifted starlight as X-rays and the cosmic microwave background as ultraviolet light due to her high velocity.