How do we know that the Universe is expanding?

How do we know that the Universe is expanding? Describe Hubble′s law, and the types of measurements that need to be taken in order to measure the Hubble constant. – Describe standard candles (cepheid variables) and how they are used for these measurements. – How does invoking Big Bang explain the fact that the Universe is expanding? – How can we estimate how old the Universe is, from its current expansion rate? – Optional — current experiments aimed at measuring the Hubble constant with extreme precision

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