Question: > 
> He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power. After making purification for sins, he sat down at the right hand of the Majesty on high (Hebrews 1:3 ESV)
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I am interested in the phrase ‘he upholds the universe’
My question is:- How could Christ uphold the universe as a helpless babe and also after he had died before the resurrection.

Answer: 
How could Christ uphold the universe as a helpless babe?

He didn't.

Correctly, Jesus, *upholding all things* (πάντα
panta) by the power of His word. Nothing to do with 'universe' and a very sloppy/biased attempt of the ESV to render the Greek into English by inserting a word that is not there.

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> whom He (God) appointed heir of all things, through whom He also made the ages (not world or universe). 3And he is the radiance of His glory and the exact representation of His nature, and upholds all things by the word of his power. Heb 1:2-3
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The context gives us the *timeline* of when Jesus became the one who, "upholds all things by the word of his power".

This power was not always his to use - being given to him at his resurrection and exaltation to the 'right hand of God' who made him heir, and *through whom* God finished the creation of man in His image. Jesus being the "representation" of God's nature.

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> Jesus is ‘the image of the invisible God’ and ‘the firstborn of all creation’ Col 1:15.
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2 Cor 4:4 ‘Christ, who is the image of God’, 3:18 ‘we… are being transformed into the same image’, Rom 8:29 also.
His Son has been appointed heir of all things. That being the case He upholds, carrying on, prevents it from falling, sinking,
By the word, #4487

Strong's Concordance
rhéma: a word, by impl. a matter
Original Word: ῥῆμα, ατος, τό
Part of Speech: Noun, Neuter
Transliteration: rhéma
Phonetic Spelling: (hray'-mah)
Definition: a word, by implication a matter
Usage: a thing spoken, (a) a word or saying of any kind, as command, report, promise, (b) a thing, matter, business

The whole operation of the universe is in his hands. The entrance and presence of sin in the universe is all accounted for by one word… He has made a cleansing of sins.

Upholds:

To hold aloft; raise.
To prevent from falling or sinking; support.
To maintain or affirm: synonym: support.

He is holding and maintaining and propelling all things [the entire physical and spiritual universe] by His powerful word [carrying the universe along to its predetermined goal]. When He [Himself and no other] had [by offering Himself on the cross as a sacrifice for sin] accomplished purification from sins and established our freedom from guilt, He sat down [revealing His completed work] at the right hand of the Majesty on high [revealing His Divine authority],

It is because that He completed the work of making a cleansing of sins and therefore is seated at the right hand of the majesty.

That is how he keeps the universe going and upholding by His declaration of His work being finished because he sat down at the right hand of Majesty on high.