284
Appendices
were electrons, atomic nuclei, photons, dark matter and various baryons,
but no stable atoms, as the high temperature would have stripped the electrons from any atom that chanced to form. Occasional density fluctuations
would occur, both higher and lower than the average density. The regions
of high-density dark matter would tend to attract particles due to increased
gravitational attraction. As plasma flowed into these regions it would become
compressed.
Compressed plasma has a high internal pressure due to the electromagnetic
interactions. Once the pressure had increased, it would drive the particles
apart, lowering the plasma density in that region, thereby enabling the dark
matter, by means of its gravitational attraction, to pull more plasma into the
region, and begin the cycle over again. What we have therefore are propagating periodic density fluctuations, which are very similar in concept to
sound waves in air. As a consequence physicists call these plasma oscillations
“baryon acoustic oscillations”.
The speed of these oscillations (sound waves) through the plasma is estimated to be 60% of light speed. As the age of the universe at the time of the
Recombination Era was 380,000 years, the maximum distance that any oscillation could have travelled in this time is 0.6 × 380,000 light years (LY), i.e.
~ 230,000 LY. This is the largest distance over which oscillations could interfere with each other. For distances separated by more than this distance, the
universe had been in existence for insufficient time for oscillations to have
passed between the separated points. As a consequence this upper limit of
230,000 LY is known as the “sound horizon”.
As we saw in Sect. 11.4, a frequency analysis of the Cosmic Microwave
Background (CMB) reveals the fluctuations present at the time of the
Recombination Era. The oscillation with the largest spatial extent is the one
corresponding to the sound horizon. However, it is not the absolute size of
the sound horizon that can be measured in this way, but only its angular size,
i.e. the angle it subtends in the sky. Assuming that space–time is flat, and
thus that the light rays are not bent but linear, a fluctuation in the CMB
the size of the sound horizon would subtend an angle of one degree when
observed from earth. If space–time is not flat but curved, the angle subtended
would be more or less than one degree, depending on the nature of the curvature. The measurements of the CMB obtained from the Planck observatory
show that the universe is topologically flat at large scales to within 0.5%. This
unexpected result is explained in Sect. 11.5.
Appendices
were electrons, atomic nuclei, photons, dark matter and various baryons,
but no stable atoms, as the high temperature would have stripped the electrons from any atom that chanced to form. Occasional density fluctuations
would occur, both higher and lower than the average density. The regions
of high-density dark matter would tend to attract particles due to increased
gravitational attraction. As plasma flowed into these regions it would become
compressed.
Compressed plasma has a high internal pressure due to the electromagnetic
interactions. Once the pressure had increased, it would drive the particles
apart, lowering the plasma density in that region, thereby enabling the dark
matter, by means of its gravitational attraction, to pull more plasma into the
region, and begin the cycle over again. What we have therefore are propagating periodic density fluctuations, which are very similar in concept to
sound waves in air. As a consequence physicists call these plasma oscillations
“baryon acoustic oscillations”.
The speed of these oscillations (sound waves) through the plasma is estimated to be 60% of light speed. As the age of the universe at the time of the
Recombination Era was 380,000 years, the maximum distance that any oscillation could have travelled in this time is 0.6 × 380,000 light years (LY), i.e.
~ 230,000 LY. This is the largest distance over which oscillations could interfere with each other. For distances separated by more than this distance, the
universe had been in existence for insufficient time for oscillations to have
passed between the separated points. As a consequence this upper limit of
230,000 LY is known as the “sound horizon”.
As we saw in Sect. 11.4, a frequency analysis of the Cosmic Microwave
Background (CMB) reveals the fluctuations present at the time of the
Recombination Era. The oscillation with the largest spatial extent is the one
corresponding to the sound horizon. However, it is not the absolute size of
the sound horizon that can be measured in this way, but only its angular size,
i.e. the angle it subtends in the sky. Assuming that space–time is flat, and
thus that the light rays are not bent but linear, a fluctuation in the CMB
the size of the sound horizon would subtend an angle of one degree when
observed from earth. If space–time is not flat but curved, the angle subtended
would be more or less than one degree, depending on the nature of the curvature. The measurements of the CMB obtained from the Planck observatory
show that the universe is topologically flat at large scales to within 0.5%. This
unexpected result is explained in Sect. 11.5.