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: 
The simple answer is that He did not uphold the universe as a helpless baby. Hebrews 1:1-3 speaks of His deity/nature BEFORE He was born as a man with a human nature.

In fact, if you read the rest of Hebrews 1 you will discover His own Father acknowledges Jesus as God, vs8. Also at vs10, "And, Thou Lord in the beginning did lay the foundation of the earth and the heavens are the woks of Thy hands.

Moreover Luke 2:11, "for today in the city of David there has been born for you a Savior, WHO IS Christ the Lord." John 17:3,5, "And this is eternal life, that they may know Thee, the only true God, and Jesus Christ WHOM THOU HAS SENT." vs5, "And now, glorify Thou Me together with Thyself, Father, with the glory which I had with Thee BEFORE THE WORLD WAS."

Lastly, Philippians explains at verses 6-8 His pre-existence as God (And so does John 1:1-3) but emptied Himself, taking the form of a bond-servant/man, and being made in the likeness of men, And being found in appearance as a man, He humbled Himself by becoming obedient to the point of death, even death on a cross." And btw, God the Father and God the Holy Spirt did not go anywhere after Jesus died as a man before His resurrection. I think it's important to put things in perspective.
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.