what did edwin hubble knowtice about the direction of travelof distant galaxies that led him to preopose that the universe is expanding?
red shift
when objects are going away from you, their frequency appears lower (Doppler effect).
lower light frequency is red, higher is violet.
when certain frequencies of light corresponding to known transitions in electron states of elements are shifted down in frequency (toward red) as seen from earth, you know the thing making the light is going away from you.
When far away sources in all directions show a shift toward the red, then everything is moving away from you and the universe must be getting less dense, expanding.
Edwin Hubble was an astronomer who made groundbreaking discoveries about the nature of our universe. One of his most notable observations was the discovery of the relationship between the distance of a galaxy and its velocity, which provided evidence for the expansion of the universe.
To understand Hubble's observations, we need to delve into his methodology and observations:
1. Measuring Distances: Hubble used a particular type of star known as Cepheid variables to determine the distance to galaxies. Cepheid variables are highly regular pulsating stars, and their period of pulsation is directly related to their brightness. By measuring the period of pulsation, astronomers like Hubble could calculate the intrinsic brightness of Cepheid variables and compare it with their apparent brightness to estimate their distance.
2. Spectral Shift: Hubble observed the light emitted by galaxies using a spectrometer. He noticed that the light from distant galaxies was systematically shifted towards the red end of the spectrum compared to nearby galaxies. This phenomenon is known as redshift.
3. Doppler Effect: Hubble interpreted this redshift as evidence that the galaxies were moving away from us. The redshift is a consequence of the Doppler effect, which occurs when an object moves away from an observer. As the object moves away, the wavelength of its emitted light expands or stretches, resulting in a shift towards the red end of the spectrum.
4. Hubble's Law: Hubble plotted the recessional velocity of galaxies against their distance and discovered a linear relationship. This relationship, now known as Hubble's Law, states that the recessional velocity of a galaxy is directly proportional to its distance from us.
Based on these observations, Hubble concluded that the universe is expanding. The linear relationship between distance and velocity implied that galaxies were not just moving away from us but also from each other, indicating an overall expansion of the universe. This groundbreaking discovery became the foundation of the Big Bang theory, our current understanding of the origin and development of the universe.